Firefight with friends in ODST. Keyword friends. Unlike lolreach where you constantly get matched with random players whose sole goal is to farm and grind for credits. Coop and the Vidmasters were also pretty fun in ODST. The last few weeks of Halo 2 was pretty fun, apart from the lag. Numerous times in the other Halo games, more than I can recollect. Customs were pretty damn fun in Halo 3. I remember all the Zombies modes and maps on Foundry. I liked things like that Gears of War inspired gametype where one guy is on a Brute Chopper and everyone else has to avoid him and break the pallets or push the crates to get to the next area.