God, I know I'm going to get hate for this, but I would have to say Halo: Reach.
*flame shields up*
I was completely turned off the multiplayer due to the fanbase who seemed to always be either completely obsessive about the game or idiotic. This really only left me with the single player and custom games with friends, since the extent of customization in firefight made it less fun for some reason >.>
The single player was kind of fun but bored me a lot since most levels, with a few exceptions, seem to be just a series of firefight matches. Plus, the story was kind of predictable with a few plot cop-outs (Kat's death). As for custom games, I could never really find another really fun game mode, just a fun challenge map every once in a while.
Overall, it felt like an average game in the end with the flaws evening out with the fun I had