I'm positive I posted an answer in this thread, but I can't find it. Maybe it was in a different thread. That post was in the pre-XIII days, though, when I still thought X was the lame duck of the series (it's pointless to pick on II). Now that I'm all caught up, the easy answer is XIII - hell, I'm frankly stunned it's not getting 100% of the votes. Y'all should be ashamed.

On a positive note, it's good to see two of my top games haven't yet gotten a vote (V and VI)!