It would have to be FFVII, although I only got to the second or third 'final' boss. In my mind, I've completed it. AC2 is taking a lot of my time atm, as I'm trying to 100% complete it, i.e. find everything, do all the missions etc. Then I've got the last Ratchett & Clank game to play (Tools of Destruction was pretty good fun, hope Crack in Time is as good!) Then there's also all the PC games I've started, NFS Shift, Borderlands, Mirrors Edge, etc.. all still to finish.
only one i've enjoyed enough to warrant a really long playthrough is fallout 3. finished it the first time quickly, but realised i missed about 2/3 of the game by doing so. still haven't finished the second playthrough after about 60 hours or something
Dragon age: origins. I am stuck half way though. Play time - 60 hours. Play time spent trying to open one piss annoying door - 4 hours and rising.