Surely the most used because of its affordablity and easy access. Coffee, energy drinks, tea, caffiene pills, etc.

I wonder what academia would look like if adderall, vyvanse, modafinil were just as accessible, or even less controlled substances that are considered to enhance mental performance like L-tyrosine, alpha-GPC, Lion's Mane mushroom, Bacopa, or Ginko.

Alpha-GPC is just choline, so you can get it by eating eggs. Amino acids and mushrooms are also quite accessible.

Modafinil is straight up better than caffeine though, which is a crappy and addictive stimulant.

"just" choline is a little reductionist, it's much more bioavailable.

I'm not sure where the distinction actually lies, but it is also considered a (generally recognized as safe) drug.

Caffeine also has a track record of several centuries.

We really, really know the long term effects.

What are the long term effects? How harmful is caffeine addiction?

no, you mean Ritalin. caffeine is a joke compared to actual stimulants.