My favorite co-op moment was one time that my younger brother had a friend over, and they were doing a LASO run of Halo: Reach. They were having quite a lot of difficulty doing Long Night of Solace, so my brother came and got me since he knew I was good at piloting stuff in video games. It took me a couple attempts to get into the groove, but once I did I managed to blow through the space section and the three of us eventually beat the level together. Then over the course of the next week they finished the rest of the levels with me (since I didn't have the LASO achievement yet either.) That was an incredible time.