The standard stuff is L-Theanine + Caffeine. For cognitive enhancement any of the "racetams" are considered proven, and you branch out into untested army drugs when you take modafinil.
I've tried each of these and stick with L-Theanine and Caffeine daily. Piracetam had no visible effect on me. Modafinil got me through 36 to 50 hour work days straight, at crunch time when deadlines were needed for an old job. Work was probably at about 60% of its best at the time – easy to get into the zone but also not quite my best. I know it's not healthy but I was also curious and happy to try it out of personal curiosity. YMMV :)