(1st choice)--pure or nearly pure methamphetamine
methamphetamine is the KING of stimulants
be on the lookout for the dextrorotatory form (made from chiral [pseudo]ephedrine, not P2P). d-meth is what you want, although d/l-meth (racemic meth) isn't too shabby, either. l-meth is a waste of time.
(2nd choice)--high dose Adderall (mixed amphetamine salts)
Adderall is about 50% d-amp by weight. It isn't as strong as methamphetamine and doesn't last as long either, despite what wikipedia has to say about Adderall having the same 1/2 life as meth [9 hours].
(3rd choice)--methylphenidate (MPH)
methylphenidate might have placed higher were it not for its short 1/2 life, which necessitates repeated re-dosing
(4th place)--MDPV
MDPV is about as potent as methamphetamine and lasts a long time but will make you psychotic faster
(5th place)--Benzedrex inhalers (propylhexedrine)--contain benzene ring hydrogenated d/l-meth. good for concentration. 1/2 life is plenty long enough. can be had over the counter at many drugs stores (such as CVS) and grocery stores!!! costs between $4 and $7 something per 250mg. a true 'legal high.' I'm on it right now, in fact.
What not to take for studying:
1. MDMA--save this for the dance floor. it's not a good study aid.
2. cocaine--no way, will not help you pay attention and only lasts a minute or two when smoked
3. a-pvp--I've only done a gram of this and I love the way it smells and feels but, overall, it has more in common with cocaine than with the performance enhancing amphetamines
4. cathinones--inferior to amphetamines, don't last long enough, don't help you concentrate