Civ V. It's a turn-based RTS, which I never cared much for before I tried Civ IV, but it can be extremely fun and engrossing. It's also pretty cheap when on sale (both expansion packs are required imo), very mod friendly, and has many different difficulty levels. Also, while it has online play, I've probably only played 5 games online in my almost 2000 hours on it, so single player is extremely viable and it's also really easy to play and do something else, like watch TV or a movie.