You all posting your most played Steam games, so I thought I'd do just as much:
Clicker Heroes racked all that playtime because it's the kind of game where you can leave it going and it'll play itself. Sometimes I even forget it's on and then I remember, bring it up, spend the accrued gold and leave it do its own thing, at least until it's time to stop idling and to start clicking.
Saints Row: The Third has been probably the most fun GTA-like type of game I played. I still want to go back and complete some achievements like the choice ones and 100% it as it should be.
Assassin's Creed II, my first AC and, so far, the only one I played, despite buying the others too (except the latest ones because they're still too high in price for me). The setting was fantastic. I mean, Italy and I'm Italian, so it was pretty cool and the story had some of history characters I like.
Middle-earth: Shadow of Mordor, the Batman-meets-AC game.
You only kill Orcs,
An Orc-killing simulator is how some describe it. Sure, you mostly only do that, but it's fun, so is it really a problem? I had tons of fun slaying those orcs, building my own army of orcs, making the orcs I marked raise through the ranks to see the carnage unfold when engaging certain leaders.
Sniper Elite III is another fun murdering simulator, though you murder people instead of orcs. Trying to kill everyone without getting spotted, planning who to take out first, trying to shoot at the right time to hide the sound, all were pretty interesting aspects of the game.
UnEpic, a game I liked from the start and that kept growing on me until I reached a boss that I (still) think is an asshole (the Eyeball one) and then I stopped there. I appreciated the humor in the game and the gameplay was fun, but fuck that boss.
Deus Ex: Human Revolution, another game I haven't completed despite sinking 32 hours into it. I remember having a lot of fun with it and then getting sidetracked by something else. What that something else was, though, I don't remember.
Dust: An Elysian Tail was such a pleasure to play. The game wasn't exactly hard and some of the animations weren't really to my liking, but the colorful look and the gameplay were good enough to keep me hooked until the end. It also made me wish for Noogy to make a Paperinik game. I'm sure it would be amazing.
Batman: Arkham Asylum GOTY made me wonder how people could stand that camera. It mostly felt claustrophobic to play for me. Still a fun game and a game I mostly played to let the young kid of the woman that came to clean my house have some fun, as he was the one wanting to play it the most.