on a single campaign/single player play through? probably LaPucelle tactics (roughly 400 hours)
In total time, with multiple playthroughs, that one easily goes to Castlevania: Symphony of the Night, which is still the best game ever made in my eyes. on my first playthrough, when i was 9 at the time of release, i believe i spent over 150 hours on it, getting the full 200.6% completion. since then, i've made a tradition to replay through it at least once a year, and i believe i have 13 200.6% completion files, and various other files that i completed, only so that i could see all of the endings i also have one filed with the 210% sword brothers glich completion. i'd say i've easily spent 1500+ hours on that game. i can do a 200.6% completion in under 60 hours now though.
For total multiplayer time, probably goes to super smash brothers melee, which i think was around 3200 hours of total logged playtime the last time i fired up my gamecube (yeah, fuck brawl)
WoW probably still beats them all...christ i'm glad i quit that game. i had 3 characters with over 100 logged days of playtime each, along with some various alts, which probably totaled another 100 days.
yeah, i'm a loser, but surprisingly not a virgin. (bi-sexuals ftw)