The Conflict games. To be more precise, Desert Storm 2 and Global Terror. The reason why? Because it was the first video game I'd ever played, and therefore, I overlooked dozens, if not hundreds of it's flaws.
It's still a decent co-op game to come back to and play when you're bored, but it takes an almost masochistic amount of patience to get through some of the levels and put up with the stupid AI.
It's still a decent co-op game to come back to and play when you're bored, but it takes an almost masochistic amount of patience to get through some of the levels and put up with the stupid AI.