Naughty Dog, EASILY. The whole Uncharted trilogy is absolutely spectacular. Sadly, I still haven't played The Last of Us.

Honorable mention goes to Rocksteady, which managed to create two of the best games of the generation - licensed games! Crazy!

Also, Sucker Punch actually impressed me with inFamous - great gameplay, and an entertaining plot with surprising plot twists. In fact, Sony's studios as a whole have produced some excellent first-party games. LittleBigPlanet, God of War III...

Sales-wise, the obvious choice is Infinity Ward. They've turned Call of Duty into one of the biggest gaming franchises ever. Considering the fact that CoD4 is not even 6 years old, it's really impressive.

If you count it as one developer, Nintendo EAD easily. The two highest rated games of the generation (and the highest rated game of all time), industry changing games like Wii Sports and Wii Fit, and all around fun games like Super Mario 3D Land (which should count if we're counting other games released in the last couple of years), Mario Kart 7, and Skyward Sword.

Sora

They made SSBB and Kid Icarus: Uprising, probably THE two games I've had the most fun with this generation. I must have put around thousand hours into SSBB alone...

I'm a fan of Twisted Pixel, Bioware, and whoever made Super Mario Galaxy. Epic didn't do too shabby, either.

Naughty Dog
Santa Monica
Kojima Productions

For the sheer number of games they put out that held my rapt attention for at least a hundred hours apiece, it's Nintendo for me. Brawl is my most-played game this generation, the Galaxies an Donkey Kong (second party) were transcendent, the Wii Sports games were brilliant for parties, and I effing loved slicing up fools in Skyward Sword.

Second place goes to Quantic Dream, whose one spectacular game (so far) single-handedly justified my $600 PS3 purchase.

Honourable mentions to:

Rocksteady!!
Epic Games
Naughty Dog (only for TLoU, though)
BioWare
Bungie/343 Industries

Naughty Dog: self explanatory
Epic: Gears alone guarantees them a spot
Infinity Ward: CoD craze started from them
Nintendo EAD1: if they where responsible for the Galaxy games
Bioware/Bethesda: took over the RPG crown from the Japanese gaming industry
EA sports: lets be honest, the FIFA series shows us how sports game are done
Bend studios: personal favorite, handheld GODS
Santa Monica Studios: amazing games, amazing tech
Valve: Portal anyone?

But all in all, I would say ND. They made sure Sony survives this gen, and made 2nd place a guarantee.

I have to go with Naughty Dog. Uncharted series plus The Last of Us... All of these games are a part of my favorites for this gen.

Monolith soft, just for the masterpiece that is Xenoblade Chronicals.

Here's my favorite devs this gen.

Team Yakuza 

Yakuza 3 and 4 are some of the best written games of this generation.  Gameplay is well done, Yakuza 4 adding three characters all who play different, then the Dead Souls spinoff basically playing like Yakuza with guns.  The amount of content each of these games has ranges from all the different quests you can do plus all the mini games that are added in as well.  Boxcelios 1 and 2, Bowling, Table Tennis, Shogi, Karaoke, Pachinko, and Golf are about half the mini games included in Yakuza 4 and all of them are very high quality for nothing more than additional side content.  There are all the Ryu Ga Gotoku games we didn't get in the west like Kenzan and the Black Panther games, plus Binary Domain is a decent third person shooter.

Naughty Dog

Uncharted is an amazing third person shooter series with a great traversal system which breaks up the monotony of just constant shooting.  Each game in some way impproves on the last, while debatably Uncharted 2 is better than the third, the melee segments of the third game add a lot to the game that the second just doesn't have.

My opinion of best developer is based on the Uncharted games as I've still yet to purchase or play The Last of Us, so can't really judge the game on personal experience, but from the official videos I've seen and from reviews, it takes the Uncharted style and pushes it as a stealth/survival style gameplay with far better gameplay than any in the same genre. 

FromSoftware

The Armored Core 4 games, Demon's Souls, and Dark Souls were all great games.  In my opinion, unless you're going to play a lot of multiplayer online Armored Core 5 and Verdict Day are disappointments and the worst in the series.