IX > X > VII > XII > VI > VIII > XIII > X-2 > XIII-2
I enjoyed all of them, but if I had to put it in order, it would probably be that.
IX was the first one I properly played, and I fell in love with it. It was lighthearted, but also had quite a grim, mature undertone at places.
X I loved due to the graphic enhancement, voice acting, story, and Rikku.
VII is just classic.
XII I loved the airships, and how political it was, especially with the Judges. I found the way they spoke under their helms very cool.
VI had Kefka.
VIII enjoyable story, but a little weak at times, I thought.
XIII beautiful game, interesting story, but too linear. Didn't feel like a Final Fantasy game.
X-2 enjoyable, but also too..."immature" at times. A little watered down, I thought.
XIII-2 similar to X-2. Story was awful a bit wank, and it didn't have an ending...
I'm currently playing through III at the moment on my phone. It's a lot of fun, although not great in terms of story, as it just seems to be a lot of random little events leading up to one big, but yes, it is fun, and quite nostalgic.