Prolintane, hands down the best functional stimulant for me, it's the desbetaketon variant of alpha-PVP (and not available anymore

), which is pretty on its own but quite euphoric, which may or may not be what you want, depending on you and what work you want to having done. Imo the euphoric stims have their place as well but as negrogesic said, serotonin releasers deplete the precious serotonin much faster than it gets restocked, but for that we have 5-HTP which is just limited by its property to induce nausea when taken w/o a peripheral decarboxylase inhibitor (they aren't to get w/o some carbidopa in which one doesn't want). 5-HTP circumvents the rate limiting step in serotonin biosynthesis so it fills up the stock more fast than naturally.
At least 4,4'-DMAR, one bomb of serotonin releaser (it was nicked 'SeroToni' for a reason; but also a weak MAOI) was surprisingly functional. I just loved doing anything or nothing but more often anything because of the stimulation. Could use it for like 5-7 days in a row at ~25mg before effects began to fade noticeably. Forgot to try 5-HTP. This is one for special events (and not available as well, argh)..
Had some N-ethyl cyclazdone from a reputable source and it was weak as fuck. Was one gram, I went through it in 1-2 days and had little effects at all. Phenylpiracetam on the other hand seems to work (it's been too long ago to say anything more about it) and 2-FMA is good, somebody who was into meth said it was weaker and less euphoric but similarly focus inducing - this individual had to do hard work on earth and it helped her.
I hesitate a bit to recommend alpha-PHiP or whatever the currently legal one is called, because this group got so bad press about "flakka" but don't believe all of that, or the meth faces / your favourite anti drug commercial. Here in Mexico they began recently to bring spots on radio: Whatever drug you take, you make damage to yourself etc.. call line 0800-xyz if you need help.. I'd bet that nobody ever uses this line. People who do drugs usually want to do drugs. So, back to the PHiP. At least alpha-PVP was nice, a good balance between euphoria and stimulation. So if the current ones are similar it might be a candidate but be warned, you need a scale for these compounds (cheaply available in that big 4-letter auction house), eyeballing will lead to dose escalation. Besides that, you want to use oral route - you could acquire empty pills off the same source and fill them with your desired amount (which is probably below 10mg, alpha-PVP was mg-by-mg at least as potent as methylphenidate, if not more.