I liked X, it had some interesting innovations. I actually sorta liked the sphere leveling system, it was pretty sweet to be able to get everyone else's stuff. Also, some of the hidden stuff was cool, like the ranch fighter thing, with stuff that was much harder to beat than the last boss. That made the game for me...and the in depth story line wasn't terrible.
But I'm with Mak, VII/VIII all the way for me. Those along with III are my favorites.
EDIT: Poizen, FF3 for DS is nowhere near the same as the original FF3 for SNES. I got excited about it when I saw it then got sorely disappointed.