I missed out on a bunch of these as they were released, but I loved 6 and 7, disliked 8, was kinda meh on 9, and liked 10.
I have a difficult time actually writing out what I like or dislike about an RPG, because what I value most is a sense of immersion, but that's such a vague feeling. 6 and 7 had a nice strong buildup, character development, and a storyline I cared about. Kefka/Seph had a strong and early showing, their appearances emotionally charged. The esper/materia systems were simplistic and very integrated with the story. The story was mostly serious but took ample oppritunities to poke fun at itself.
8 felt jumbled up and I had little empathy for what was going on... I didn't care about emo Squall or his little band of supermercenaries or whatever the final boss was doing (I don't even remember). It was too serious. 9 took out a lot of the bad bits in 8 but didn't replace it with good stuff so it was pretty bland. 10... had a hint of a mystery about it that I felt interested enough in figuring out. I didn't care about most of the world, or the people, but that starting hook carried me through just to see how it all turned out.
FFT has to be the final fantasy game that drew me in the most. The character development was immense and the plot was surprisingly deep with politics and religion and aspects of the caste system. The job system was fun and encouraged experementation. FFTA disappointed me greatly though I understand that the market was different.