Borderlands, (the first one). It's quite fun and the world is really big. I could in theory sink hundreds of hours into this and enjoy all of it. I've got the game-of-the-year edition so all the content is available. The growth and progression of my character is enjoyable as are the varied and multi-level monsters and villains. I'm not a fan of the jumping puzzles though. I've already purchased Borderlands 2 but who knows when I'll get to play that.
I'm also trying out some beta versions of some games. This week it's Landmark which is a sprawling game and not so easy to play in a casual way.