Mine are strange ones that you wouldn't expect to spend so much time on, longest would be
1) Half-Life 2
Back in my teenage nerdy days I used to count how many times I finished the game and was proud of it. I believe it was something like 43 times when I stopped counting.. no joke. Plus, adding in all the time I spent on the Deathmatch (god I used to be amazing at the game, never lost) and we are probably into the 1000 hour mark.
2) Mafia
Did the same thing for this game (I really have changed now.. lol) and I think this was also about 30 times, the game is longer than HL2, but no MP to waste more time with, and the Freeride modes I never cared for.
3) Mass Effect 2
...probably. I've finished all the ME games more than once, but ME2 clocks in at 4 times. 4 x 30 hours = 120 hours. For a recent game and my current lifestyle that's pretty hefty, and its the only thing that comes close to the other 2.