In terms of what the game clock says... Probably FFXII where I've clocked in close to 200 hours on it, and I still don't have any character higher than Lv. 80.

Yet overall, I'm going to say FFVI and FFVII, I've probably replayed through both of these games more times than the rest of the series. I still have a fairly good memory of both titles despite my last playthroughs being a few years back. I can proudly say, I've uncovered most of secrets of both games without a guide, just plain old trying out new things and experimenting. Hell, I'm still learning new things about each game when I play them today, and when I read up on stuff. They are probably the only games where I actually have fun reading through mechanics guide and pages of accessories and enemy lists.

Though if I was going by a Square RPG overall, Chrono Trigger has probably received the most love from me of any RPG.