FF 9 wins my vote for being a combination of all that came before, and having a fun story and sidequest set to boot.
However, I own almost all the FFs out for the PSX including Tactics, Origins (I and II), Chronicles (IV and Chrono Trigger), Anthology (V and VI), VII and IX, and have X and X-2.
For a mindless sequel, X-2 is ok. I think it was loads better than VII, II, or Mystic Quest. Mystic Quest could only be second worst in my mind, right above VII.
And at least X-2 returned to the ATB system. I missed it's dissappearance since 6.
On another note, I wish that Squeenix would re-release Seiken Densetsu 3 and package Secret of Mana with it. Then they could join Legend of Mana in my collection.