Well, personally, I have only played I, VII, X and X-2 (among the complication of Final Fantasy VII and Kingdom Hearts) so I can't really say for sure.
Meh I'll just vote X for the heck of it.
...You know, for some reason I never liked FF. I played FF4 on the GBA, and thought it was stupid how you'd lose precious party members. I like the Fire Emblem better because it (arguably) has better plotline, and you can use many more characters.
I've never even played final fantasy. the only FF game I ever played, was kingdom hearts one and two. they were made by the same company that made FF.
Final Fantasy - Boring, I prefer the next one...
Final Fantasy II - Remade on the PS1 as FFIV, but this is still good.
Final Fantasy III - Don't think I've played this one before...
Final Fantasy IV - Pretty good. Probably the best.
Final Fantasy V - Never played.
Final Fantasy VI -Never played.
Final Fantasy VII - My brother LOVES this one.
Final Fantasy VIII - Never played.
Final Fantasy IX - Never played.
Final Fantasy X - Played, still half-way through, average.
Final Fantasy X-2 - Very good, could do with male playable characters but those babes are HOT! Ouch!
Final Fantasy XI - Never played, I have a slim-line PS2, I think this is the only game it can't play.
Final Fantasy Tactics; or Tactics Advance - Good, but could be better, slightly.
Any of the Crystal Chronicles - 3 letters- B-A-D
Other (includes Complication of Final Fantasy VII, Chocobo series, Ehrgeiz, Kingdom Hearts, and others) - Y'wot? I've only heard of Kingdom Hearts from those, but they have DISNEY characters in like MICkEY MOUSE and GOOFY. Not good.
From that, it's GOT to be FF IV.
*is betting Kero voted for FFXI*