Well guess most will say their own servers
Well
http://flyff.gpotato.com/?m=rankings #58 Joceninja is me...so Aibatt
and ye well i will help you on your way from my experience
it all depends what your after but to some short info about the servers....
Demian it has PK system(really awsome) and 2x exp cause of that but pretty empty
Aibatt and Lawolf the most users playing at also the oldest servers
rest is pretty much equal but i would advice Mushpoie if u dont wanna go aibatt/lawolf/Demian...prolly the best server of the other they got good raid teams...lots of ppl using forums pretty much what u need =O