Halo was super popular in my area growing up, and when it came out my family didn't have an Xbox yet. When we got it, my brother and I made sure to convince our dad for Halo CE to be our first game (because of the Mature rating, and all). Not only that, before this point we've never played an FPS! We forced ourselves to learn how to play just to experience the game. And the split screen co-op was a dream come true for that. I think it was the first game that got us up and shouting for doing something epic, and the first time we were scared from a game (thanks, Flood). And we created our roles for future games as well. Whenever there would be a vehicle with a driver seat and a gunner position I was always the gunner, and him the driver. It's become an unspoken rule at this point.
Another solid string of memories are from high school, between the years 2006 and 2010. This is around the time Halo 3 came out, which was my first delve into online multiplayer. Me and the guy I consider to be my best friend had an epic Competitive Slayer match that I remember to this day. We were Blue team in a 4v4 match on a map we've never seen before, and as soon as the match started the other two on our team dropped out. Us, not being quitters, decided to just roll with it, and I tell you we've never been this tactical and talkative while playing. We had each other's backs in the most dire of situations. When one dropped, the other would hide out until they spawned and we would regroup ASAP. We never picked different targets, always focused on the same one so they could drop quicker. We held out til the end. In our reluctant 2v4 match up, we lost 50-49, just ONE POINT SHY of victory from just the two of us. Not only were we impressed with ourselves, but we got a lot of Xbox messages from them saying "guys that was epic", "great game", and "holy **** you guys". To this day, I consider that to be my greatest Halo achievement