What do you mean by "best"? Recreational or functional?
Functional: 2-fma is definitely worth a try. I found it much more cerebral oriented as opposed to effects in the periphery, like levoamphetamine has. Better for things that require concentration rather than physical activity, although I didn't try it for the later much so I may be wrong.
For non-RC's: Caffeine + l-theanine is very popular in the nootropics community and recommended for beginners as their first nootropic. May sound underwhelming, but it actually is a very good combo.
Nicotine is also one of the most potent cognitive enhancers, with insignificant detrimental health effects on its own. Read the pages by gwern and examine if you're interested.
As for recreational stim, all the legal ones are shit, don't bother
Bullshit, 4-fa was generally very well received.
Any newer RC stims that have come out and are worth trying? The market can move pretty fast, so I don't know if anything worthwhile has come in the last few years that I'm not aware of.