Don't have one.
Halo 4 and ME3 multiplayer would be mine BUT i don't pay for Live Gold anymore.
On PC, i have DOTA2, MechWarrior Online and PlanetSide2 but i play each only rarely. Don't have enough patience nor care to master DOTA as fun as it is (shit game for casual MP gamer as i'm nowadays), MWO's issues pushed me of the game for now, and PS2 i started again today but i wonder how long i will keep playing it. EDIT i also have SC2 WoL but i never got into it, it is not fun like Warcraft 3 was. Too big armies for me.
I'd play EVE Online or some other MMO but can't afford them right now.
So, no go-to MP game. What i want doesn't exist. Nor will anytime soon, for it is too niche.