FFVI was definitely the one that got me the most... I played that game through at least 20 times, although I also liked IV... it had a few of the best characters IMO (Kain and Rydia). I'll be honest though, I was a diehard Nintendo fanboy back in the day, and when FFVII was released on playstation I boycotted it...
It wasn't until FFXI was announced that I finally broke down and bought a PS2.... and decided to buy the FF library for PS1&PS2. I liked 7, but wasn't as blown away by it as others were... I hated 8 (not really sure why to be honest, but just couldn't get into it).... then I played ...9 and played it again... and again... not sure why I liked it so much... maybe it was the story, maybe it was the humor, it just clicked with me and I truely enjoyed it.
I never finished 10 (I found it far too cinematic for my taste... and thus never played 10-2), played 11 for more years that I care to admit (Beta through to the end of last year) and dabled a bit in XII and XIII (I really like XII, but never had the time to really sit down and enjoy it... XIII made me angry after about 8 hours of straight line style play). I recently bought XIII-2 just because it was somehting like $10 lol, but haven't loaded it up yet, as I want to finish XIII first to see how it ends.
tl:dr
Loved FFVI and FFIX, played the snot out of FFXI and currently enjoying FFXIV and hoping for good things with v2.0
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95THF, 95DRG, 90BRD, 94BLM, 95BLU, 90COR - Retired: Nov 2011
Someday soon my friends, this ride will come to and end, and we can't just get in line again.