I can't pick one, so for this poll I voted for V.

I know someone mentioned selecting and combining abilities in Tactics, V set the whole foundation for that idea. Great job system, memorable characters, first appearance of Gilgamesh, first FF that really made you think about how to build your characters, first FF with alot of sidequests at the end, last FF with Crystals (not counting VII), last game directed by Sakaguchi, first FF with Kitase & Nomura, alot of reasons.