Bungie - For Halo: Reach. Breath-taking originality it wasn't, but I still sunk countless hours into multiplayer.
Gearbox - Borderlands. Fresh, fun, and original. It didn't try to play grim and realistic. Plus, pony friendly. More tears, please.
Ubisoft - Assassin's Creed. Still managing to be fun and I haven't regretted time spent on a sequel.
Bethesda - Skyrim and Fallout aren't really my thing, but I respect the scope and other reasons why others find it so amazing.
Infinity Ward - Call of Duty. Still massive and powerful, regardless of what you think about their games. Again, countless hours purely in multiplayer. Still haven't sunk a minute in campaign.
I like my multiplayer spunkgargleweewees.