If it is from your list of games, I would go for Just Cause 2 for the pure insane fun it can be and Metro 2033 for a beautiful experience of a destroyed world and also a challenge on higher difficulties
Should try out Garry's Mod a bit more if you haven't already. Game modes like TTT and Prop Hunt can be a lot of fun. Maybe play Half Life 2 if you haven't already? I heard it's a pretty amazing game. TF2 Rocket Jumping has kept me quite busy too.