Batman Arkham City, Just Cause 2, Borderlands
I have all the games and love all of them, but Borderlands really isn't very fun if you're playing by yourself. It's one of those games where coop is the core experience, and without that, you're missing out on the entire point of the game.
Just Cause 2 is one of those games that's great for messing around, but that's about it. I mean, it's a fantastic game, and all the mechanics are solid, but it's also one of those games where you really have to make your own fun. The missions and story aren't nearly as worth while as just causing random mayhem.
Batman Arkham City is the most structured experience out of the three, and definitely has the best story, which is the biggest thing I look for in a single player game. It's also fairly long, and quite a lot of fun, with tons of secrets and collectables to find, and a new game plus mode if you want to extend your playing time. Don't play it without playing Arkham Asylum first though.