Both KotOR games. - We all know why.
Dragon Age Origins - Well, I actually have this on every platform and have done complete, thorough playthroughs on each, but the real time-killer was the PS3 version, as I pushed to get the Platinum trophy, and I clocked 300+ hours on that, last I checked. So I think in total about 400 hours was spent playing this game.
Terraria - One of those games that once you start it's hard to let go, but then once you've done everything you want, you never really play it again.
70+ hours according to Steam.
League of Legends - The thought of figuring out my playtime on this scares me. Brilliant MOBA.
Burnout: Paradise - I have no idea why, because I was never that much into Racing games, but I was a sucker for this. It was SO fun and I clocked at least 50+ hours on this.
The Mass Effect Games - Before Mass Effect 3, I played this series all the damn time. The first game in particular, NEVER got old for me, and I could just keep replaying playthrough after playthrough. I must have clocked 100s of hours on this series. Mass Effect 3 just sorta lost all reason to keep playing the series.
Skyrim - According to Steam, 347 hours. Needless to say, I love Skyrim. Trying out different combat styles, doing thorough playthroughs.