Halo. I've played missions here and there, but I don't think I've beat every mission, much less in order. Tried playing them for the backlog challenge some time ago, but the first Halo campaign is so slow, wide open and takes a but to make progress. I'd like to do co-op campaign for the games that have it, but planning it out is another thing. One day.
Multiplayer is where it's at for me in Halo.