For me, its Pokemon - the entire main series must have taken up well over 300, maybe 400 hours of my life altogether, and I haven't even finished Platinum yet.
Other ones that have taken up a large portion of my life were Dragon Quest VIII - clocked up 90 hours play time on that - Final Fantasy VI, VII, X, X-2 and XII, and Fable.
Oh, and the Metal Gear Solid series, although a lot of that has been watching the cutscenes in-game feature-length films.
Also, the Kingdom Hearts games have stolen large portions of my life as well.