Top 10:
1. FF10 - Easily my favorite. Loved the strategic take on the battle system, the Sphere Grid was amazing, and the story sticks with me to this day.
2. FF9 - The best PS1 FF. Great battle system (if too slow) with personality and charm to spare in the characters.
3. FF6 - Not without its issues, but a true classic for sure. Still has some of the best music in the series.
4. FF13 - I loved it despite its issues and think it gets way more crap than it deserves. I got attached to every character, and loved the linear, narrative-focused experience (the 20-hour battle tutorial, not so much). Deal with it.
5. FF13-2 - Both better and worse than 13, but ultimately 13's longer, more focused narrative wins the day for me. Loved both however.
6. FF8 - The best soundtrack in the series, and a departure for the franchise in terms of story and scenario. When it works (not always), it really works.
7. FF4 - An essential part of JRPG history, and should be played by anyone who loves the genre. It's showing its age though, and its legacy has been diluted through so many re-releases.
8. FF10-2 - Amazing battle system, rollercoaster narrative and world structure. Some of it's great, some of it's not, but it was a successful experiment.
9. FF7 - Will always be credited for putting JRPGs on the map in the U.S., but has aged the worst of all FFs post-3 and is antiquated and overrated in its current form. Could be amazing if remade, though.
10. FF5 - Never could get into this one. Definitely gets credit for introducing the Job System, but uneven difficulty, bland characters, and a completely forgettable story ensured I never enjoyed this one much.
The ones that didn't make the cut:
11. FF1 - The original. The foundation. Completely antiquated by today's standards, but remakes have rendered it playable and helped it remain relevant.
12. FF12 - A massively multiplayer offline RPG. I don't enjoy MMOs, so I didn't enjoy this. The License Board was a horrible idea that was terribly executed. One of the series' worst main characters and a totally forgettable narrative just cements FF12's place near the bottom for me. Balthier was awesome, though.
13. FF3 - Not a terrible game, but just completely antiquated by today's standards with a meh story, and overshadowed by every other FF game except...
14. FF2 - Worst. Battle. System. Ever. Also: wildly uneven difficulty, and a whatever story. Pass.