It's a close tie between FF Tactics Adv 2 and FF Crystal Chronicles:Ring of Fates. The depth and unlockables of Tactics is awesome. But the artistic design and incredible multiplay options are superb on RoF. So, if you are more into turn-based strategy games (like Advance Wars and Disgaea) get Tactics Adv. If you are more into the standard adventure, rpg games (like Zelda and Soma Bringer), and especially if you like playing with friends, it's FFCC:Rof all the way.
P.S. Coming in at third place would be FFXII:Revenant Wings, if you are into real-time strategy games (like Robocalypse, Summon Night, etc.)