I believe that very much depends on your priorities.
Most of the "good" cathinones are already banned in most of the US and Europe, with N-Ethyl-Hexedrone ("HEX-EN") supposedly being the only worthwhile one left.
2-Fluoro-Amphetamine and 2-Fluoro-Methamphetamine used to be fairly popular (since they were very close to regular amphetamine) but they got banned in China, so the supply has pretty much dried up. Same with 4-FA which was also a serotonin releaser (i.e. more of a mephedrone-like "party drug").
4-Fluoro-Methylphenidate (4F-MPH) is basically a more potent form of Ritalin... if you like Ritalin you'd probably enjoy this one... note that unlike 4-FA the 4-Fluoro-substitution doesn't cause any serotonin release, so it's really more of "functional" stim.
3-Fluoro-Phenmetrazine (3-FPM) is also pretty popular with some users on this forum; not necessarily a functional stim though, and apparently fairly more-ish.
[Disclaimer: These are my impressions from reading through various threads on the subject. I take my prescribed ER Ritalin mostly for anhedonia and lack of motivation, as I am not into an endless cycle of euphoria, binging, crashing and fiending, so take my summary with a grain of salt).