I enjoy Halo's storyline and campaign, and their multiplayer fills a certain niche of non-realistic first person shooter that is fun.
CoD has a bit of a ridiculous storyline, but it is enjoyable like an 80s action flick. The multiplayer is thinly treading the line between realistic and non-realistic that it is annoying. It can tote itself as a realistic shooter, but the sniper rifles don't kill you quickly, you heal from the strawberry jelly in cover fairly quickly, and a grenade which has a lethal range of 15m won't kill you from 15m. Obviously some of these are because a perfectly realistic shooter would be hard as balls.
BF3 had a rather cookie cutter campaign which is only really memorable if you realize that the multiplayer is the aftermath of the campaign, and that the protagonist loses. Doesn't happen often. But their multiplayer is definitely fun. I feel that they do add a bit more realism with the multiplayer, with added physics engines and Frostbite, however, they still aren't too realistic, but as I said, a perfectly realistic shooter would be hard as balls. I enjoy their multiplayer because the games feel immense. The matches are huge, the availability of equipment allows for variety in gameplay, and the guns feel more... gunlike. I never really feel like I'm shooting marshmallows in BF3, compared to CoD where I often feel like my gun is less lethal than it should be.
Escape will make me God.