Valve.
Portal 1 & 2; L4D 1 & 2; TF2. No-one else has come close. I don't actually think they've put a foot wrong this generation (if we count the generation from ~2007-2013)
Special mentions:
Naughty Dog: But I haven't played The Last of Us yet so it's still a while until I'd nominate them for the top spot.
Nintendo EAD Tokyo: I don't know whether this is actually two separate studios, but whoever it was who made Mario Galaxy 1 & 2.
And just a few more I'd like to bring up:
Insomniac Games: It feels weird listing them considering they disappointed me hugely in the past 3 years (All 4 One; Full Frontal Assault; Fuse). But that's ignoring the fact that they put out 3 excellent Ratchet games (Tools of Destruction; Quest For Booty; A Crack in Time). And I had a lot of fun with Resistance 2's co-op mode & Resistance 3's single player too.
Gust Corporation: This might just be because I'm playing Atelier Ayesha at the moment, but I feel like they've made 3 excellent JRPG's in a row now (Totori; Meruru & Ayesha). I haven't played Rorona & Ar Tonelico Qoga yet though; which might sour of my opinion of them.
Studio Liverpool: WipEout Pure; Pulse & HD Fury. 3 fantastic returns to form for my favourite videogame studio of all time. And technically all part of the 7th gen. But their output could've been a lot better, so they don't hold a candle to some of the other teams I've listed.
