Best type of meditation for attention

ADHD like any other disease has origin in imbalance of five elements. Meditation is a word derived from medicine but unfortunately it is not the complete cure for a disease. It is part of balancing oneself. You need to detox through diet and lifestyle changes prior to starting meditation. I suggest you take a look at Pancha-karma of Ayurveda or Accupuncture and massage therapy of TCM. If you have resources go to India or China to get the best people. If you want to go for fast relief take veg light diet and deep tissue massage for 15 days (start with light massage) on a continuous basis.

Meditation is of two types - centering, which requires you to concentrate and decentering which requires you to let go of mind. I believe silent witnessing will work best in your case. Just sit or lie with your spine straight and watch. Do not try to control your body, thoughts or emotions. Use music, incense for help.

I'm relatively new to meditation, but a few things spring to mind from my experience within one tradition:

From my experience, I'd be very surprised if one form of meditation will suffice (this is true for most people). You may need a combination of awareness/mindfulness meditation to cultivate undistracted (while unstrained) concentration, kindness meditation to allow whatever arises in your experience to open up in its own way and time, and to give yourself the confidence to continue, and a receptive meditation such as the zen zazen or just sitting practice to encourage openness and patience with your experience, and an unforced approach.

Choose a set of meditations which work well together. If you are a beginner, it might be best to find a particular system or teacher and stick with their practices. I can offer the Triratna tradition's meditations, with free guided meditations here: https://www.freebuddhistaudio.com/audio/ (search "metta bhavana", "mindfulness of breathing", and "just sitting" - I would, but my internet's playing up)

Stick with the practices rather than jump around between traditions/styles. This will allow you to cultivate momentum and depth in your practice, whilst avoiding confusion. ONE CAVEAT: this is unlikely, but if the meditation is putting you in a worse state of mind, STOP (though unpleasant isn't necessarily bad). Discuss your difficulties and problems with someone, and consider changing teacher/style.

You will greatly benefit from a lifestyle which compliments your meditation, otherwise you will be pulling in two directions at once. For example, try

• spending time outside of meditation without occupying yourself with activities (i.e. stillness), e.g. sitting in a chair without a purpose (not reading, talking etc) - even if this is only 5 minutes a week at first, increase it over time. If you forget - that's fine. I suspect this might be the most important one for you (though it's difficult to tell without meeting you)
• extending your kindness meditation to generosity, patience etc. outside your meditation
• finding things that you naturally enjoy focussing on undistracted - perhaps landscapes, relaxing music etc

And if you can, I strongly urge you to find people who are also practising meditation to discuss the process with in person.

You should also try these Meditations

Breathing Meditation :- Help increase the focus and calmness of a mind

Meththa Meditation :- If you are an angry person this will help you to come over it

Meditation of posturest full awarness :- Increases Mindfulness

... I was wondering what kind of meditation works best for creating lasting focus and a higher attention span in the mind. ...

Anapana meditation increases focus and attention.

Scientifically demonstrated benefits

See also: Research on meditation

The practice of focusing one's attention changes the brain in ways to improve that ability over time; the brain grows in response to meditation. Meditation can be thought of as mental training, similar to learning to ride a bike or play a piano.

Meditators experienced in focused attention meditation (anapanasati is a type of focused attention meditation) showed a decrease in habitual responding a 20-minute Stroop test, which, as suggested by Richard Davidson and colleagues, may illustrate a lessening of emotionally reactive and automatic responding behavior. It has been scientifically demonstrated that ānāpānasati slows down the natural aging process of the brain.

(source: https://en.wikipedia.org/wiki/Anapanasati#Scientifically_demonstrated_benefits)

So does Metta meditation

His mind can concentrate quickly

(Source: Metta (Mettanisamsa) Sutta: Discourse on Advantages of Loving-kindness)

You might want to try Samatha meditation. This type of meditation would have you fix your attention on a single object of meditation over a period of time. The common object is your in and out breaths. The point of focus is usually the tip of the nose or the upper lips where you could feel the in and out breaths making contact. Since this training requires you to stay with a single object over a long period of time, it could help your mind maintains a higher attention span and lasting focus on a subject. Sutta reference is available here and a detailed analysis is available here. Good luck..