Prince of Persia : Warrior Within is up there. Must have played it at least 15 times. Don't really know why, it's not THAT replayable. But at the time, all this dark, edgy, emo stuff with "hard" music and super sexy chicks that wear nothing but ridiculous armor bikinis just hit my buttons as a teenager. I would have stopped after the 8th time, but then I found out there was a secret final boss.
Guild Wars may be lower on the list. Played it forever and never got good at it, and certainly not rich. Still have no clue how everyone else made it. Some obnoxious kids and a stalker finally ruined it.
Much more time went into the Freelancer multiplayer, must have played it over the course of two years. There was a clan war in which I played a major role. The server was at war, which continued in flame wars in forums and admin rage. After it settled down, I missed something in that game.
Fallout 3 is high up there, and had I more time, Skyrim might take it's place. I'd say there are at least 600 hours in FO3 and FNV. And probably whole days just searching for mods for all of them, reading books worth of text just to get them to work, and lots of customization, modding mods.
And Team Fortress 2, as already mentioned, with 650 hours. Minus ~120 hours of idling, plus 60 hours or so to get it to work. It might be the most frustrating and most fun game of them all.