For me, in order:
- IX - This was the very first FF I've ever played, and I loved it. The characters aren't overbearing or too cliche. The story and plot themes are excellent, and I love how the game never takes itself too seriously. The weapon/armor-based character progression is the best in any FF I've played to date. I still haven't beaten it all the way (last boss gives me fits). Also, Vivi is the best FF character ever!
- XII - I just love the overall "fantasy" feel of this game. For me, this game was a paradigm shit from all the other FF games I've played as it was less about the main character themselves, and more about what was going on in the world around them. However, with that said, Balthier is the true lead in this game and it shows. While the OG License system was a little too bland for my taste, the International Zodiac Job System blew this game wide open for me.
- X - Basically, this game took the best parts about the characters from IX, and gave them voices. Was never to fond of the Sphere Grid, though. And the cutscene with Yuna and Tidus in the lake/pond = classic. X-2 can go surf on a wave of dicks, though.
- VII - I just like the steampunk feel of this game. And while I don't care much for Cloud, I love Tifa, Garrett and Red XIII. And messing with Materia combinations was pretty cool.
- XIV: ARR (PS3 beta) - I know it's too early to put an unreleased, unfinished game in my Top 5, but unless something drastic happens between now and August, I'm pretty sure this game is gonna take up a lot of my time. It has the feel of XII, and has fan service everywhere. That's good enough for me.
With all that said, I'd put XIII waaaaaaaaaaaaaaaaaaaaay down at the bottom of not only my FF list, but near the bottom of my JRPG list, with only Infinite Undiscovery being a bigger failure. Don't get me wrong, as a genre-neutral standalone game, XIII can hold up well. But it's not a good mainline Final Fantasy game, and it's not even a good JRPG.
That's why I'm so worried about XV. While it was a definite megaton at E3, the more I watch the trailer, the more XIII-vibes I get from it.