If one FF game has VERY hard (or at least hardest) bosses which is almost impossible to beat, then that will be the main point which makes the leveling system more awesome. You will get motivated easily because of the hard bosses, it makes you think of your best strategy, the spells make you more exicted too... everything else.

Or that's at least what I think. I mean if one FF game has easy bosses and has a great battle or leveling system then it will be boring.

Anyway, I pick X because exactly what I said, it has the hardest bosses.