1, 2, and 3 I never could get into. Dunno why, they just never caught my attention.
4 was and will always will be my personal favorite. Didn't like the surprise end boss who came out of nowhere though.
Liked 5 a lot. Won't say loved, but liked a lot.
6 was great. While it wasn't -my- favorite, it's the best FF game ever made, period. One of the best villains of all time, hands down.
7 was good, but it was overrated. Overhyped, overadvertised, and overemphasized. In the end, it was the first 3D Final Fantasy and happily sported one dimensional characters. The only one who had any decent sort of character development was the only one to get killed off (Phoenix Down anyone? Anyone???) and the final villain was too obsessed with "mommy".
8 was okay but the draw system ruined it. You have to draw 300 of everything, and then never cast spells? Lame.
9 was pretty fun and I'd happily replay it anytime.
10 was okay. Yuna and Rikku were both WAAAYYY too happy and just annoyed the **** out of me; I couldn't stand it and gave up around Macalania. Tried again later, gave up again for the same reason.
10-2 was ALMOST as bad but I suffered through most of it, following a 100% completion guide. Missed .1% somewhere by the last chapter, couldn't find where. Gave up and quit.
11 I still play. Has its faults, sure, but I don't see myself quitting anytime soon.
12 I liked. Reminded me of an offline FF11; all the fun without the retarded noobs, NM competition, or AH price gouging.
I actually liked MQ for what it's worth.
FFL and FFA I liked as well.
FFT was one of my favorites by far. I admit though that I use the "Never lose JP" cheat from game shark whenever I play though.