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