9 for the references to past and I like the characters/light-hearted dialogue.
6 because the story is awesome and also Locke is awesome.
5 because the job system is very fun.
10 because Lulu and Rikku are awesome and I like the sphere grid.
12 because I like the gameplay, though I wish the spells looked cooler and the international zodiac job system was released ... internationally
13 because it's fucking beautiful

Haven't played any of the other ones. Except 1 and I always die before I get to Garland.