1. Reach, I played this for three years non-stop. Summer 2011 gave me some of my best memories of online gaming. And I guess it's really the game that got me into online gaming. I met a lot of people playing this game, and got my first group of "gaming buddies", good times.
2. Halo 3. Loved the story. Didn't really enjoy the muliplayer at that time, but I suppose I was only around ten years old. It was actually my first Halo game (well, not really, I had downloaded a pirated version of Halo:CE which had one server which was 24/7 Bloodgulch) and was also my first videogame I really enjoyed.
3. Halo 3: ODST. After fanboying over Halo 3, you can imagine the raging hard boner I got from playing this with my best mate; whose was also, suprisingly similar to mine, Aiden.
Col. Preston Sloboda set Min-kyu Seong's custom class to Irish Republican Army.
Halo Combat Evolved and Halo Two even though two had a lot of shitty parts that weren't fun, but it had a lot of story involved which puts it up there for me.