Far Cry 2, I'm starting to compare it to Assassin's Creed. Its got really repetitive single-player (there is multi-player, but no one ever plays it so it might as well not even be there), but you can have fun just running around killing people. There's all sorts of fun weapons, and if you buy the Fortunes Pack for $5, you get 3 free weapons: Silenced Shotgun, Sawed-off Double-barreled, and a crossbow with a scope.......that shoots explosive arrows.
And then of course there's CoD 2/4/WaW (3 wasn't for PC). 2 is still fun, I played the missions in that game literally every day after school in middle school. 4 was great, advanced the single player to the next level (with modern warfare. And despite what people say, WaW was still a good game. Good single player and multiplayer. I actually like WaW multiplayer more than 4 multiplayer.