Okay, I'll play.
Behind Civ 4, The Sims 3 is easily #2. Besides CIV I've played it more than any other game. A lot more, in fact, though it's still not even close to how much time I've spent with Civ 4. Sims 2 is fun too, and I still play that one from time to time. I have no intention of ever touching Sims 4.
Fallout: New Vegas is a more recent favorite of mine, but I love it. I still haven't even gotten to the end with a character in it yet, but that's part of its charm for me. It's a world I can enjoy and be satisfied with even without ever reaching the end of the game.
Sid Meier's Pirates, a really fun game every once in a while
Dragon Age: Origins was great, although it's not really as replayable as I hoped it would be.
Age of Empires III is honestly not very good, and I prefer the classic version of Age of Mythology. BUT, it had a wonderful scenario editor and the community used it to create games that were far more exciting and enjoyable than anything from the standard game. Colosseum, Maze Runner, and especially Cannibal Escape, all fantastic. If I had to guess, I'd say it's my third most played game. Thanks almost entirely to those multiplayer scenarios, though. I think I played the demo more than the actual game.
Honorable mentions go to the Tomb Raider series and to EU3, even if I'm embarrassed to play it most of the time.