Final Fantasy 10.
my experience with FF games before that was a brief spell of FF1 and then FF9 a few years later. so i was all like "how this a final fantasy game, its all set in the future and stuff.. gawsh, i'm off to go grab bums as zidane and be scared of heights as Vivi!"
a few years later, i stumbled into it again and needless to say, its the most hours i've put into any final fantasy game... infact, if you get all the other amounts of hours from the other games, add them together, you probably still get less than the 300+ hours i spent in that game... keep in mind i've gotten to the point on FF4 where i could kill the last boss with any selection of characters by just pressing attack for each one... and i played FF9 to death a bit too.