IX without question. It's my favourite game ever in general. The rest of the games are a bit more difficult to put in order. Of the main series I've played every single game through except II, XI and XIV including the sequels to X and XIII, as well as most of the games related VII. I've also played a lot of the spin-offs, like Dissidia and Type-0.1.
I guess my top 5 would go something like this:
Though besides IX, the rest of them could be in almost any order, depending on the day.