Grand Theft Auto 4 (Awesome story, and the add ons are coming to PC soon, so its getting on for 100 hours of game)
Red Faction Guerilla (Insanely fun, and you can take the world apart with your hammer)
Mercenaries (The original, mixed opinions on this but I thought it was excellent fun)
Elder Scrolls IV: Oblivion (Massive World, great fun especially if you’re into RPGs. Get the story out of the way and get stuck into the side quests)
Out of them, GTA4 is the unmissable one. The depth and detail of the world is incredible. If your PC can’t run it, GTA San Andreas was also incredibly fun and easily the second choice
EDIT: Just realised how many times I wrote ‘fun’ in that response :mono: