Dota 2; that being 393 hours and still counting!
31 hours on Source Filmmaker. That's a game, right? ...right?
Looking at my Steam listings... I think it was Darkness II at 10 hours. That's not actually very long. I've only played the tutorial of XCOM and only 5 hours of Sleeping Dogs but I fully expect that if I finish either one of those games it'll rocket me forward. ...for comparison, Serious Sam 3 is the game I played most last year, at 22 hours. ...I don't play many games for very long, if I'll be honest.
Diablo 3 with 160+ hours followed by Dark Souls PtDE with 140 or so hours. Dark Souls wins though since I had at least as much on 360 as well. I don't think any other games where close this year. Except League, but one does not simply compete with that, I don't want to know this years playtime in that game.
If we are talking about a 2012 game, probably Diablo III at over 60 hours followed by Mass Effect 3.
If we can include releases from the year before, Skyrim probably beats every game ever for total time spent, and that would include just in calendar year 2012. 180+ hours total with probably half of that, or maybe more than half, in 2012. That game just makes you waste time in it.
Crap, I forgot time spent in MMOs, so yeah, Star Trek Online is high up there this year as well.
1) Civilization 5 Gods & Kings at 464 hours
2) Sonic Generations at 95 hours. Yes it came out in 2011, but most of the time spent playing was this year.
3) Torchlight 2 at 58 hours
4) Dust: An Elysian Tail is around 40 or so hours (beat it twice at 15 hours a piece, working on hardest difficulty now)
5) Borderlands 2 at 37 hours
There's others as well, but those are my biggest time-sinks this year.
@Animasta: Diablo 3 by far; kind of a waste of time though I don't really regret it since it was a fairly unique experience to do Inferno ridiculously early and at some point I did have a character worth several thousand dollars though I'm not the sort to sell; granted it's worth about 50 bucks now.
AC3 multiplayer up to about 60 hours now, 10 odd hours in single player so far.
100 some hours in SWTOR since free to play