Hmmm, a tough question, since the series has changed a lot over the years, and several of the games had really different, yet outstanding qualities.
The first I played was good old IV (don't worry, I won't yell at you for not putting it as an option ), and I have a lot of fond memories about that game, since it was really my first RPG too. Simple yet effective fantasy story, diverse characters, often hilariously awful translation, it really had it all. It also had Rydia, the first fictional character I ever had a crush on. (*Adult* Rydia, for your information)
FF VI, on the other hand, was a very different game in a lot of ways, yet equally good. The translation was better, the characters were more three dimentional, and it starred one of the most memorable FF villeins ever. On the down side, I think there was really a lot more characters than could reasonably be developed, and several of them felt sidelined or outright superfluous. The Esper system was neat, but not very well developed.
Ahh, FF VII, the most popular of the Final Fantasies. While I didn't like it quite as much as the previous two, it really had a lot going for it, not the least of which was the most complex and layered FF plot yet. The characters were well done, and the visual effects were truly spectacular for their time. Omni-Slash is *still* the most badass FF sword attack.
Now FF VIII... I wasn't really a big fan of FF VIII, the characters seemed pretty two-dimentional, the plot was needlessly weird (Time Compression!), and it lacked any kind of decent villein. The draw system could have been interesting, but ended up being kinda tedious instead. Still, the (non-drawing) combat was surprisingly fun, and it did feature a blonde female teacher who wore glasses and fought using a whip. Which *was* awesome.
FFIX, while being an overall excellent game, there were a few things about it that bugged me. I enjoyed the characters (even though Amarant and Quina felt tacked-on), and the skill learning and equipping system is *easily* the best progression system in an FF. I also liked all the little references to FFI, and the music was superb. The only thing that detracted was the main villain, Kuja, wasn't really up to the standard of Kefka or Sephiroth, and the game really deserved someone of that caliber. Also, the final boss made less sense than Ultimecia. Still, I liked it a lot.
FFX. Well, there was some good ideas in there, but man, Tidus was so annoying it made it *really* hard to enjoy the game. Also, the equipment and combat seemed like a step backwards, though the idea of summons actually participating in battle was a good one.
I know a lot of purists are going to want to stab me, but I really liked FFXII. Good characters (even if Vaan was way to similar to Tidus for my comfort) with excellent voicing, and the first FF plot since VI that actually made somewhat sense. The MMO style battles weren't nearly as bad as I had feared, and were actually quite tactical, at least in the early part of the game. The license system, however... not so good. Kinda bad, in fact. I should never have to fight *that* hard to keep a group of six characters from becoming statistical copies of each other.
In answer to the poll question, I liked most of them, but for very different reason. Also, Karbo, sorry for the giant essay here, I kinda got carried away. I should probably just expand it a little and make it into a journal entry or something...
Final Fantasy VI will always be my favorite of the series, with Final Fantasy VIII coming in at a very close second. See, FFVI was my very first video game, overall. I first played it when I was 5 years old, and that was 15 years ago (yes, I played it not long after its release). I've grown up with the game, and I've grown closer to the characters over time, as well. Locke has always been my favorite male character throughout the series, and Celes is my favorite female. I also enjoyed the immense cast of playable characters, which is still the largest in the series to date with 14 (counting Mog, Umaro, and Gogo). And the fact that each of those 14 characters had a backstory that impacted the overall storyline (except for Mog, Umaro, and Gogo) made it that much more appealing. Not to mention, you can't find a crazier villain than Kefka. You have to love his iconic (albeit annoying) laugh. ^_^