Nine was probably my absolute favorite! They ditched the dark side of Final Fantasy and went to the series light hearted roots. The CG was nothing short of spectacular. But the thing that really won me over about this one were the time consuming side quests. I'd literally spent over 20 hours on side quests alone to get every item in the chocobo treasure hunt and although the final boss wasn't as great as i'd hope, the battle with Ozma was EPIC.
X was close, but I liked 9 a bit better. 7 was excellent, but overrated in comparison to all the other gems in the series.