If we're counting legal drugs here, probably nicotine, because it doesn't even produce a high once you've been using for like a 3 weeks.
Otherwise, I dunno.
After about 8 years of drug use and at least 4 separate addictions, I've come to the conclusion that all drugs, even the ones that supposedly improve performance or whatever, come with a price they just ain't worth.
I'll say this though: there was a period in my life when I was deeply in debt, depressed, socially isolated, etc. During the period when I pulled myself out of that rut, I was prescribed ritalin and snorting it. No, I don't give the ritalin credit for my rebound, but it didn't exactly ruin my life. That said, there was a reason I stopped getting those prescriptions refilled.
Anyway, with regard to psychoactive drugs that supposedly improve performance, I think that the performance enhancing properties are generally overstated. Like, there are definitely drugs that improve focus and shit, but you mega-focus can be directed toward a videogame just as much as your homework or whatever... right?
Here's what I think of the 'functional' drugs that I've tried:
Methylphenidate (Ritalin) - The improved focus and motivation is definitely there, but its effects are too short lived to make a positive difference over the long term--even if you ingest it orally. Also, ritalin creates a very strong compulsion to re-dose, if you're prescribed it, it's easy to get locked into a binge where you keep doing it for days and days on end. And the drug changes considerably after about 5 or 6 re-doses, you just start feeling really spaced out and numb. At that point, productivity suffers.
Meth - I've only done meth once, but the sense I get is that you'd be pretty productive after the rush wore off and were still stimulated, but you'd likely be unproductive after you'd been up for 30+ hours.
Adderral (Dextro and Levo amphetamine) - I'll admit this stuff definitely makes it a lot easier to focus on just about anything, and unlike ritalin, it does help you 'choose' what to focus on instead of just going all in on whatever you'd be doing anyway. The thing is, it just keeps you up way too long; after a while that takes its toll.
Cocaine - No dice. Too short lived and you're too miserable during the comedown to do anything.