I would say Just Cause 2. The PC port of GTA IV is quite bad, so you probably don't really want to play it unless you have a very powerful computer, or an Xbox 360. Mafia II is okay, but there isn't much to do in the world aside from the main campaign, which isn't all that long. It's an average third person shooter with some driving mixed in.
Just Cause 2 is a very expansive sandbox, with plenty of stuff to do. Lots of stuff to destroy, and lots of places to explore. Using the grappling hook to do all sorts of crazy stunts while destroying all sorts of government property is quite amusing. The voice acting is pretty bad though. If you want to play the PC version, you will need to have Windows Vista or higher, as the game only runs in DirectX 10, so no Windows XP support.