All of them...I love each and every one equally.
But if I had to pick just one, it'd probably be FFVIII. I don't care what anyone says about the storyline being stupid, I think it's great. I adore the junctioning system, and being able to use it to complete the game at level 20. The music was on par with that of FFVII (my favourite musically) and really helped to give the game that much needed feel.
The only thing I didn't like about the game, was the the inevitable love interest dominating the game. It hasn't been so bad in other titles in the series, but in FFVIII it was crammed down out throats to the point where it felt like:
"Yay, after 4 discs we finally admit to loving each other...now lets go kill Ultimecia!"
But I can forget about that, thanks to FFVIII holding my joint favourite mini game in the Triple Triad card game.