If we are talking about overall gametime, then FFXII (over 160 hours now and I still have to complete the Pirate's Den and beat Yiazmat.)

If we are talking about games in the series I have devoted my time to through frequent play through's; then FFVI is in first place cause I absolutely love this game and I generally play through it at least once a year.

FFVII is the second game I've devoted the most time to but I played this in order to understand why people like it. I was horribly disappointed in it the first time I played, then dealing with the obnoxious fans made me start to really hate this game. After a few years passed and my anger subsided, I started to play through it a couple of times with more realistic expectations. I think the game is pretty good, not the best the series has ever offered but at least I can finally understand the appeal a little.

FFIV is the next game I've played through countless times, my first FF I actually owned and completed. I still love the characters and enjoy watching the journey of Cecil and Kain.