What is the most noteworthy defection of a member of the Nazi party to the Allies during WWII?

Goering was the only one. Obviously the war was over by then but Hitler was still alive and ordered his Assassination. Ironically he was considered Nuremberg's "top catch" even though besides the Battle of Britain he was mostly a political figurehead. Whether Admiral Canaris and even Admiral Raeder were in fact traitors is still speculation. The Nazi Command Structure had far from total control over the totality of the "Nazi Regime."

I miss one person on this list: Admiral Canaris. Like the others, he wasn't a defector. But he greatly hindered Germany's war efforts, because of his dislike of Hitler.

He was completely trusted by Hitler, and send by him to Franco to negotiate a treaty between Spain and Germany. Either to allow safe passage of the German armed forced to occupy Gibraltar or become a direct Axis partner.

Franco didn't want to support Germany in any way. Instead of negotiating for Hitler, Canaris told him exactly how to do that without incurring the wrath of Hitler.

That's exactly what happened during the meeting in Hendaye between Hitler and Franco. The meeting failed. Afterwards Hitler remarked he'd rather have a tooth pulled than negotiate with Franco ever again.

Canaris was head of the Abwehr, and did a lot to foil Nazi plans, but always indirectly. He was found out, arrested, tortured, put in a concentration camp and executed under humiliating and torturous circumstances.

Try to consider Willy Lehmann. He was recruited in 1929, before Hitler came to power, initially for money, but when the War became expected to start, his collaboration became more ideological as he was strongly against it.

He headed the Gestapo department that was doing counter-intelligence against Soviet industrial espionage.

He transferred over a lot of very important info to the Soviets, that on German weapons, rocket technology, submarines, Gestapo cryptographic codes, and even the exact date for the invasion of the USSR.

When the USSR had lost link to him, he conducted a risky step, and in 1940 put a letter into a post box of Soviet embassy calling for continued collaboration and said his work in Gestapo would lose any meaning if the USSR would not resume contacts.

In 1941 after invasion of the USSR the connection with him was lost. In 1942 the Soviets tried to reestablish contact, but the Germans captured the sent communicators and executed Lehmann afterwards.

I'm not sure whether being captured blunts the definition of "defection" at all, but after Field Marshal Friedrich Paulus' capture, he cooperated with the Soviets completely, even making anti-war propaganda statements for the Soviet Union.

In April 1945, toward the end of the Nazi regime, Heinrich Himmler, the head of the Gestapo (Secret Police) and "SS" military units, left Berlin against Hitler's orders, and began negotiations with representatives of neutral Sweden, as an attempt to surrender Germany to only Britain and the United States, while allying with them against the Soviet Union.

He did this on his own authority, without the consent of Hitler (who expelled him from the Nazi party), in fact, against Hitler's express instructions to stay in Berlin. His overtures were rejected, but he was basically the highest-ranking Nazi (fourth after Hitler, Goering and Goebbels) to try to negotiate with the Allies without authority from Hitler. (Admiral Doenitz, who did surrender to the Allies, did so with authority vested in him by Hitler's last will and testament.)

Operation Sunrise led to the surrender of all Axis forces in Italy on 2 May 1945, 5 days before VE day. Several high ranking officers were involved in the negotiations and the story is complicated, but Field Marshall Albert Kesselring, the Commander in Chief of the Mediterranean Theater, knew of the negotiations and ultimately approved - I would call this a 'noteworthy defection.'

Allen Dulles' book 'The Secret Surrender' tells the story.

How about (for an EARLY example) Fritz Thyssen? He was a wealthy industrialist who gave large contributions to the Nazis from 1923 to 1932. He was one of the major characters who urged von Hindenburg to appoint Hitler chancellor and was elected as a member of the Reichstag. However he soured on the Nazis (and Germany) at the beginning of WWII and took his family to Switzerland (and then France), after which he was expelled from the party and his company was seized. Later he made the mistake of going to Belgium right as the Germans invaded and he ended up in a concentration camp, but managed to survive the war.

Defection rather implies changing sides in a conflict. None of the senior Nazis seem to have done that:

• Hess seems to have wanted to persuade the UK to make peace with Germany, but his idea of how to do this was hopelessly wrong. He seems to have assumed that the UK monarchy of the 1940s worked like the pre-WWI German Empire. Richard J Evans' The Hitler Conspiracies: the Third Reich and the Paranoid Imagination, Penguin, 2020, which is about debunking said conspiracies has an account of the offer. Germany would get back the colonies taken from it at the end of WWI, but the British could keep their empire if they gave Germany a free hand in Europe, and made peace with Italy. The British were not at all interested in this: if Germany were given time to consolidate its conquests, it would grow stronger, make new demands and a new war would break out.

• Himler was trying to preserve his existing position as head of the SS and Police, and add the leadership of Nazi Germany to it, by getting the Americans, British and French to join the German side in the war against the USSR. This was considered treason by the existing Nazi government, but was not a defection to the Allied side.

• Surrendering yourself, or the forces under your command is not defection. If German forces in Italy had switched sides and made war against Germany, that would be defection, but they didn't do that, and the Allied side wouldn't have been interested anyway.

• Willy Lehmann was a spy, but spies aren't usually considered defectors unless they visibly change sides, or flee to the side they've been working for.

• Paulus might be considered a defector, at the point when it was starting to become clear that the Germans weren't going to win, but he was a military officer rather than a politician, and not a Nazi Party member.

Ernst Hanfstaengl quit the cause before the outbreak of war, but it's hard to call him a senior Nazi.