I haven't played any FF after 9 so this poll isn't really for me, but out of the ones I did play I liked 8 best. It might have had a few stupid little plot twists and an ending that came out of nowhere and only started making a little bit of sense in hindsight, but in many ways I prefer it over any other FF game
The music, the characters, the setting... The soundtrack is one of the best FF-soundtracks ever (One Winged Angel can go suck a fat one). Seifer is one of my favorite recurring sub-bosses, because unlike clowns like FF5's Gilgamesh, FF6's Ultros, or FF7's Turks (who are all crowd favorites, presumably because they're clowns), Seifer and his crew actually had very personal motivations for antagonizing you, and you actually ended up feeling kinda sorry for them (I was really glad for them being alive and happy in the ending cinematic). Triple Triad was a lot of fun once you got a feel for it (and used the evolving ruleset to weed out a few of the more annoying rules). The Junction system made the game way too easy if played right, but was very enjoyable to use, especially since how much of it you used was pretty much entirely up to the player (I once played through disk 1 entirely without junctioning anything). The Guardian Forces finally evolved FF summon attacks beyond "an extra strong and fancy-looking attack" and the Limit Breaks (while easily abusable) added a level of interactivity to the fights that was previously unseen in FF games.
It was certainly a flawed game, but it's also one of my favorite jRPGs of all times. Whether this means anything coming from someone who claims not to like (most) jRPGs is up to the reader to decide.