My fondest memory is how people I knew who weren't gamers became gamers after playing Halo CE and Halo 2 single player co-op with me after school...everyone always fell in love with Halo because it was more than a game, it just had something special about it that made it different from every other game out there at the time.
I remember one time on a Friday after high school let out, my one non-gamer friend stopped by my house to try out Halo 2's campaign with me and wound up having to call his parents to tell them he'd be staying over for the night because he wanted to finish the campaign with me. We literally stayed up all night long finishing the campaign on Legendary, fell asleep on the couch with controllers still in hand, and couldn't stop talking about that night for the rest of the year. Later that week he calls me to tell me he bought his own Xbox with Halo and a ton of other games. Halo 2 converted my old friend into a lifelong gamer.
That's one of my fondest memories, on top of playing customs everyday after school for like seven hours straight.
I kid you not when I say this, but the custom games in Halo 2 brought me so much joy for so many years that Halo 2 will always be in my heart. The memories that I have with that game are amazing and I have not experienced anything like it since with regards to gaming.
My other memory is how I came into Halo to begin with. I was at the local video game store with my mom looking to rent something, but I didn't know what. Nothing made me happy like Goldeneye N64, so finding new games was kinda bumming me out because nothing compared to that game for me at the time. I remember my mom calling me over and saying something along the lines of "Hey, look at this game called Halo, it looks pretty cool. Why don't we rent that for the weekend?" I looked at it in awe with pictures on the back and Master Chief on the front..."Halo" just felt so different in that moment. I agreed instantly and that Monday we went back and bought a copy because I could not stop playing it. I was OVERJOYED to learn Halo 2 was coming out and would be playable online...Halo 2 literally started Xbox Live for the most part, so the online community was awesome for the 7 years or so that it was online.