X for me, without any doubt. Why? Because I think it's the best in the series. X-2 has great gameplay and optional challenges, but it put you into these "oh my god this is stupid..." moments (Massaging Leblanc anyone?) and I think the story was nothing special and it might've even hurt the Spiran mythology somewhat. But then again, I have to give it props for having the best optional dungeon in FF (IMO).