I would have to say no, the Soviets did not excel in any single weapon. They did have some very good weapons, and a few were superior to their enemies equivalents in any given period of time (eg T34 in 1941) but over the course of the war, only one weapon truly excelled and that was the atom bomb. The Soviets excelled not because of their weapons (no country did) but because every time a soviet soldier died, they could put another one into the battle in a uniform and with a gun. Given that the Soviets had between 8 and 14 million military and possibly as many as 24 million civilian deaths during WWII, that was an awful lot of bodies.

When evaluating a tank you have to consider a few factors besides firepower and armor to get the real performance. For instance: ergonomy. T34's was awful. That means reduced rate of fire and crew underperforming due to exhaustion. Ability to see the enemy. If you don't see an enemy tou can't hit it. T-34 slits didn't allow to see much. Optics. and accuracy of the gun. If you don't hit you can do little damage. German tanks were superior. Ability to operate in commander exposed mode. It is dangerous but makes the tank far more efficient. The T34/76 or at least its early models couldn't. How it takes the hits. The armor of Stalingrad-made T34s easily cracked and sent lethal fragments of metal when hit. That was because it was not a single piece of armor but several pieces cast together. Crew survivability if destroyed. If tank easily catches fire and is difficult to exit, if ammo easily cooks up and explodes you will lose a lot of crews. Experienced crews are precious and more difficult to replace than tanks. T-34s didn't shine in this area even T34-85s. Effectiveness of their ammo. Soviets considered the Sherman 76's gun, (not the Firefly who was in class of its own but the American-made long gunned versions) as equal to the T-34/85's despite firing a smaller, lighter shell. My guess is Soviet ammo was made of poor quality metal. Weight of the round. IS-2 was so heavy that it came in two parts. A panther could fire 3 times well before an IS-2 could fire one time. Tigers too had a much higher rate of Very, very, very important: a radio to warn your comrades they are walking into an ambush, a radio to coordinate the action of your platoon and mount ambushes of their own. A radio to coordinate with other arms. I think T34/76s never got one as standard equipment

Then there are the strategic aspects. How much maintenance it needs? A Tiger required eleven hours for an hour of combat. How many miles before breaking? If enemy is attacking 60 miles from where you are and your tanks break after 30 you are in trouble. Panthers and Tigers were notorious for their unreliability. That could be one of the reasons Soviets favored multi-pronged offensives at a a couple days intervals: to force Tigers and Panthers "to run" from one fire to the other. Can it be repaired quickly and easily? Is it light enough to use about every bridge or will it be forced to use only a few selected ones thus allowing the enemy to foresee your moves? Is it a fuel hog? Road and cross-country performance. A slow tank like the Tiger will often arrive too late.

Last but very important: How much it costs? If tank is expensive to manufacture you will have few of them. and they will be outnumbered.

So despite their armor and guns Soviet tanks were not as formidable as they looked once you integrate tactical factors. But despite the apparent strengths of German tanks, once you integrate the strategic factors like ability to move from one side of the front to the other or their cost they weren't that good. Sure, they destroyed a lot of Soviet tanks manned by crews far inferior to the German ones but if they lost the war it is for a reason.

PS: If my memory is any good in 1945 the Red Army was losing less men than the Germans. Anyway the exchange rate had dramatically dropped since 1941 and 1942. I don't have numbers for tanks but there is reason it had aldo sharply decreased. Anyway in the 1945 offensives you see time and again the same pattern Germans ksocing a few tanks, survivors retiring and then Katyushas obliterating the German tanks or PAK that were causing trouble. Rinse and repeat. Nehring's crack Panzer Divisiwon was so badly mauled by Soviet artillery it was totally powerless to do anything against the advancing Soviets and was forced to a desperate retreat for its survival

The Soviet advantage in winterized equipment was critical. Lubrication that didn't freeze, winter camouflage, clothing that actually kept you warm.

When you say "in WWII" you miss the point that WWII lasted 5.5 years. And all these 6 years there was intensive arms competition on both sides. Soviet tanks T-34 and KV where by far the best tanks in 1941. No other country had anything comparable. (See the Schwern answer for details). But in 1944 this was already not the best tank: Germans tanks were superior. Soviets also had some superior airplanes: superior when they were first used. Examples are IL-2 and Yak-3. Another very effective weapons was the famous unguided rocket system BM-13, known as "Katyusha".

References: Wikipedia articles on BM-13, IL-2, Yak-3, Kliment Voroshilov tank (KV).

One thing that has yet to be raised was propaganda and espionage. The importance of this was noted particularly in WW2 and the Spanish Civil War which were wars of ideology as well of substance.

The Soviets had by far the most successful spy setup in the world, partially due to their claim to legitimacy as a union of socialist republics. This lead to huge sympathy among workers in many countries and of course, intellectuals. The Soviet spy rings penetrated British, United States, German and Japanese targets with virtual impunity. To the West and the Germans, the Soviet Union was an enigma wrapped in a riddle; to the Soviets their enemies were as transparent as glass.

This of course was supported by a huge propaganda effort aimed at perpetrating the myth of the 'Workers Paradise' helped not only by the Soviets but by 'useful fools' in every country of the war. Even the phrase 5th Column comes from Soviet instigation.

This is not to say that the Soviets acted wisely on the information they were given. Often they ignored it or interpreted it in terms of Socialist Doctrine rather than objectively but the capacity to gain information was by far the best and most effective that existed in the world.

The T-34 tank was the outstanding armament produced by the Soviets during the war. It came as great surprise to the Germans and significantly outclassed the German tanks. The T-34 was an important factor in blunting the drive on Moscow during the Autumn of 1941.

Although the T-34 had first appeared on the battlefield as early as July 1941, it was in too few numbers to be noticed and fully appreciated. This changed in October when it appeared in large numbers for the first time with decisive results. The tank was markedly superior to the Panzer IVs which the German's had. On October 10th a large formation of T-34s suddenly appeared and mauled the right flank of the German army group involved in the Tula offensive. This critical action slowed the advance on Moscow and was a factor in delaying the Germans and saving Moscow, thus ultimately forcing the Germans back into a defensive posture that winter.

When the Germans debated how to answer the tank problem, one proposal was to simply copy the T-34, but this was not adopted for various reasons. For example, the T-34 had an advanced water-cooled, diesel engine made partially out of aluminum which it would have been difficult for the Germans to copy since they had more limited supplies of aluminum. Ultimately, they created the Tiger tank which was superior to the T-34, but produced in much smaller numbers, so that the sheer volume of the T-34s continued to overwhelm the German forces.

Schwerm is correct when he stated that the T-34 produced an international revolution in tank design when it appeared in combat in '41 during the initial months of operation Barbarossa.

But I disagree with his assertion that the T-34 and KV-1 were the "best" tanks then in existence. Yes, the best tank designs; but operationally, due to poor Soviet engineering, they were extremely unreliable and had appalling rates of mechanical failure. In addition to poor optics, the turret design of the T-34--where the commander had to assist with main gun operation, and had extremely poor vision compared to German designs greatly reduced their combat effectiveness. Until 1944, because of these shortcomings, a T-34 was able to fire only 1 round for every 3 a German crew could fire. Which explains why German tank aces like Michael Wittmann could destroy dozens of T-34's in a single engagement.

The trend in tank design during the war towards ever larger tank chassis, with ever thicker armor, requiring ever larger armor-piercing main guns--this was a constant for the Germans ever since the '40 France campaign, when their Pz.II's with 20mm L55's ran up against French Somua's. By March '41, Rommel's Pz.III's with short 50mm guns had difficulty against the better-armored British Matildas.

But the T-34's appearance later in '41 created a revolution in armor not only because of heavier armor and a 76mm main gun--a combo that totally outclassed the German Pz.III's and Pz.IV's--but because of its design: the first medium tank with sloped armor and a coil-spring (Christie) suspension with a wide track base that gave it the best surface:weight ratio of any tank then in existence. Which, in the mud of a Russian Spring/Fall was crucial to operational mobility. As Schwerm also said, it didn't fully come into its own until the T-34/85 version, with a larger, three man turret, an 85mm gun with better optics, and a host of lesser improvements.

While the Soviets produced more powerful tanks, like the late-war IS series, the T-34 dwarfed all of them in total production. At over 64,000 units, it was the most produced tank of the entire war, and the second most produced in history.

The Panther Sd.Kfz.171--which appeared in '43--was arguably an almost perfect, German copy of the T-34. How the T-34 "could" or "should" have been; using sloped armor, a 75mm high-velocity main gun with high quality optics and a wide track base. It took the Germans another year to work the bugs out of the Panther design and get it as good as it was going to get, by which time the reliable T-34/85 was entering service.

The German Tiger I heavy tank program was greatly accelerated by the appearance of the T-34--but it was never really intended as a replacement for the Pz.IV's, which ended up as the most common German medium tank of the war. A single Tiger I required almost twice the resources of two Pz.IV's or Panthers or Stug-III assault guns. The Germans, like the Soviets, recognized the prhobitive resource cost-restraints of an entirely heavy panzer force.

The Katyusha BM series of mass-fired rocket launcher was another game changer that also revolutionized warfare, and like the T-34 was also copied extensively by the Germans.

Although it's not really about "weaponry", yet I should add something that others seem to forget completely: the sniping. Although the Red Army lacked good sniper rifles, yet it had an absolutely superior sniping school.

One may easily find dozens names of German aces of Panzerwaffe and Luftwaffe, but in the snipers list the whole top is occupied by Soviet marksmen.

The Katyusha. Hands down. Stalin's Organ.

The T-34 and Soviet tanks were produced in tremendous numbers -- and destroyed in tremendous numbers. The same can be said about most Soviet weapon systems. The skill and dedication of their operators was heroic and legendary, but generally their heroism was short-lived as they went up against superior equipment. Have a look at the Wikipedia entry regarding the Battle of Kursk, if you want to dispute that: Soviet losses were three times Nazi losses, despite the Soviets being the defenders in the initial stage and despite the western allies supplying the Soviets with a complete briefing of what was coming down the pike from the output of Colossus/Enigma/Bletchley Park.

But the Katyusha, now that was a different story. Cheap, mobile and deployed in vast numbers, the geography of the Eastern Front led to tactics that made them highly effective, despite their limitations. They had nothing resembling accuracy or high fire rates, so they were gathered into vast batteries for saturation bombing of a particular area and then rapidly dispersed for reloading. Rinse and repeat. The only effective defense was airpower -- destroying the formations before they could fire -- and so the Katyusha did their work on evenings, nights, early dawn and inclement weather when airpower couldn't find them. Tanks are front armored for defense against other tanks. The rain of Katyushas was both effective and far less deadly to its operators than trying to go head-to-head with Nazi panzers.

The Katyusha was a weapon that forced its opponents to change their strategy and tactics because of its effectiveness. When brought to bear, the Nazis could not concentrate and entrench their forces to defend because the Katyusha could, would and did destroy any such attempts. Given that massed panzer assault was the very heart of Blitzkreig, the Katyusha was monumental. The T-34 and other Soviet weapon systems did their work through sheer superiority of numbers and attrition. The Katyusha did it through the tactical use of a weapon system that reached the battlefield in unmatched numbers and units that remained intact and trained long enough, with commanders brilliant enough to use it to its fullest potential.

Caveat

Wars (especially in 20th century!) are won by nations/armies, not weapons. A weapon can be excellent (e.g., Me-262) but it will make no difference because it was introduced too late and/or was used incorrectly tactically. Another example is Tiger - seemingly an excellent tank, but very complex and expensive, so fewer than 2,000 of them were built (against 84,000 of T-34 and 49,000 of M4), thus, in fact, it was a bad weapon ("net loss" - the Wehrmacht might have been better off with 10,000 Pz-4 instead of 2,000 Tigers).

Therefore my list includes the weapons that made a difference, not those that were superior to the analogues used by the others.

USSR

T-34

Despite the legends spawned by the German generals right after the war, they met and successfully fought T-34s throughout the summer of 1941. IOW, the Red Army failed to exploit the advantages of T-34 (due to poor training of the troops and abysmal structure of MechCoprs). In the Fall 1941 the Red Army ambushed and decimated a German column (which, apparently, did not even send a recon unit ahead) and Guderian repeated the tale of his subordinate which blamed T-34 for it (to save his skin - hide his own complacency which was the cause of the defeat). This is the source of the canard that "the Germans only met T-34 in the fall".

T-34 had a lot of problems (poor visibility from the turret, lousy transmission &c) - nevertheless it was a net win.

The strongest points of T-34 were that is was not just a tank, it was a platform, it was being upgraded throughout the war and was still useful even after it; as well as its suitability for mass production.

Il-2

The numbers built - 36,000 - speak for themselves. WW2 was the industrial war; mass production of weapons (and soldiers who can use them successfully) was the key to victory.

Katyusha

This was not unique as an item (cf. Nebelwerfer), but the Red Army discovered The Right Way to use the weapon tactically: use a lot of them together. The unguided rockets have very low accuracy, so a few of them do little damage because they never hit anything. With massed use, the target is the area, and everything within it is eliminated.

PPSh

The importance of this weapon was, again, caused by the tactical use: a sub-machine-gun company (рота автоматчиков) in every infantry regiment.

Cf. the use of MP 40 and Thompsons which were mostly issued to NCOs.

War Management

Not exactly a "weapon" per se, this was more important than any actual weapon.

Continuous Mobilization

This was how SU replenished its army as it was being decimated by Germany: men were drafted into the army all the time, as women and adolescents were drafted into the industry to replace them.

Economic mobilization

SU immediately stopped virtually all civilian production and evacuated a significant portion of its industry.

In other words, SU started Total war right away, as opposed to Germany which waited for Stalingrad.

US

1. M1 Garand
2. North American P-51 Mustang
3. M4 Sherman
4. Logistics - unsurpassed.

Germany

Type VII Submarine

Good sub, nothing revolutionary, but together with the Rudeltaktik it made a difference - it sustained the longest campaign of WW2.

I omit the truly revolutionary Type XXI because it arrived too late to make a difference.

MG 34 and MG 42

Again, the critical part was the way they were used tactically, not just the excellent design.

Stuka

Nothing spectacular as an aircraft, but truly amazing on the battlefield: used as "flying artillery" together with tanks it was the symbol of the Blitzkrieg.

Others

Both Japan (Zero and Yamato) and Britain (Spitfire) had interesting designs, but they were not very consequential (except for, probably, Ultra).