So, so many to choose from;
Drunkenly playing Bomberman for 6 hours straight until 5 in the morning, laughing so hard that our French flatmate was banging from his bedroom wall into the living room and we couldn't hear him from hysterical laughter.
Me and a friend unlocking Mercenaries on resident Evil 4 for the first time; much screaming and shouting at every swarm of enemies that snuck up on us, and multiple failures from collapsed hysterics.
And finally, for anyone who's ever played Full Throttle, I leave you with the following line: "You can't Beat a Corley!"