Best places for levels are Bahamut's cave, Crystal Tower (Kum Kum trick) or the ??? cave. But the ??? cave is quite tough, because you only encounter the powerful Yellow, Green or Red Dragons. Best start training there only if you're level 70 or above. I'm level 80 now and it goes very fast. Faster than the Kum Kum trick anyway, and there's always a chance for Onion gear (I have one full set already, yay!). =)