Suprisingly, Half-Life 2. I replayed it about 45 times (at the time it came out I was a bit of a loser

) so I would say I put at least 150-200 hours into it, and that's not counting however long I spent in the Deathmatch (maybe 30 or 40 hours)... Yes I'm serious, I memorized every single word every single character said, and to this day I can still recite all the scenes from memory just because its been imprinted onto my brain from how many times I played through the bloody thing. I did speed-runs learning every nook and cranny and how to bypass certain areas with grenade and gravity-jumping... Ask me something obscure about Half-Life and I'll give you an in-depth answer from all the stuff I learned back when I actually cared. I still love the Half-Life series but I have my own life now, not just a half life (badum-tish) and I kinda left my nerdyness behind me as I grew up.
Then probably Oblivion in second place... It was around the same time of my life and it was my very first real RPG game, so as you can imagine it impressed me a lot.. I put about 100 hours into the game. These days I don't even think its that good.
At this point I rarely put too many hours into a game before moving on to the next one, and don't find (or want) nearly as much time to game. I just see it as one of my hobbies now but it used to be
my life >_<