1. FF5 ; Probably the most entertaining gameplay in the series, I love the more lighthearted tone of the game, the awesome customizable Job system, the soundtrack and the whole "multiple dimensions" setting.
2. FF4 ; Was my favorite game ever as a kid, the soundtrack was mindblowing the first time I heard it, and the game has such a memorable plotline and cast of characters. Plus it introduced the ATB battle system. 5 character parties is something I wish more games in the series had.
3. FF7 ; Another game that totally shocked me upon release, it has had a huge backlash ever since it was released as the game that "sold out" to the masses, but that's hogwash. Its production values were unrivaled for the time, it had a ton of optional stuff like minigames, the Chocobo races, obscure secrets to find. The setting is extremely distinct and memorable, the storyline was probably my favorite in the series too - themes like ecoterrorism, anarcho-syndicalism, cloning, genetic engineering, psychological issues... you never saw that shit in a videogame before FF7 and even if the storyline suffers from a bad translation, the point still gets across.
4. FF3 ; My first FF was 1, and my favorite in the series is 5... so naturally I'd love the in-between game. I played this for the first time right before 7's release, in Japanese on an emulator. What jumped out at me was how massive an improvement the game was over 1, and the ridiculous number of secrets and sidequests for an 8-bit RPG. The scenario writing is genius and far better than most 8/16bit RPGs, especially the whole situation with Goldor chaining your airship up. Music is stunning, graphics look great for 8-bit (especially the monster designs), now if only there wasn't that massive final dungeon with no save points to deal with...
5. FF6 ; Not as much of a fan as others are of the game, sure the storyline is great, music is one of the greatest in videogame history, and characters are extremely memorable. But I never particularly cared for the Esper system or the World of Ruin (its ugly and bleak aesthetics or its extreme nonlinear nature).
6. FF9 ; Graphics and setting are stunning, the return to "classic" FF was something really nice to see after 7 and 8, and the references to other games in the series made me smile. The characters were a welcome change after the dull FF8 ones, the charm is also through the roof on this one. Wasn't a fan of the slooooooooow pace, load times, or music. And that keeps this from being one of my favorites.
7. FF1 ; My introduction to the series, so I have a ton of nostalgia for it. The game was lightyears ahead of DQ1 (the first RPG I ever played), though it's not known for its story and is pretty ball-busting hard, it's still pretty enjoyable on a gameplay level. I love the sprawling dungeons and slight nonlinearity the game has. Modern remakes fixed some of the problems with the original, namely the enemy targeting system.
8. FF13-2 ; Just finished my first run through this, now wrapping up some of the fragments I missed and some of the Paradox Endings. Really enjoyed this though, it's got its problems like crappy balance and some annoying optional puzzles (FF has to really quit it with annoying optional content)... but it was a total step in the right direction as compared to 13. The storyline actually really worked for me, I loved the surreal nature of the plot and there was one part in the game (chapter 5) I felt to be very powerful and gripping. The music was another element that was really great, though I understand why those things don't work for everyone.
9. FF10 ; Another game in the series that oozes atmosphere, and had one of the best battle systems in the series (finally, a return to pure turn-based battles... with character hotswap!). But this game was extremely linear compared to what came before it, had some absolutely absurd requirements for lategame weapons, and had a couple of really annoying cast members. Intro was probably my favorite in the series though.
10. FF13 ; Don't really care for this game much due to the extreme linearity, horrible weapon upgrade system, and lack of all that extraneous content that makes RPGs so fun and immersive (minigames, sidequests, towns), but damn it looks pretty and I loved the soundtrack. And the battle system was an interesting experiment, switching between different roles to defeat enemies and turning even the "popcorn" battles into a little puzzle.