Best: Naughty Dog, they made the first game I ever played, then they made the greatness that was Jak and Daxter, and then they made Uncharted.
Insomniac Games as they made the original Spyro series, then they made Ratchet and Clank, then they made my first ever PS3 game, Resistance.
And finally Pandemic Studios, seriously they made Star Wars: battlefront, Destroy All Humans! and best of all Mercenaries: Playground of Destruction. 'Nuff said.
Worst: Bethesda after Morrowind. Oblivion had so much less than Morrowind, and Fallout 3 screwed up the fallout series in many ways. Skyrim better be good! Plus Bethesda seem to have no connection to their fans. Seriously Josh Sawyer answers so many questions he's asked, even ones that question things he did in New Vegas. Did Bethesda ever explain the things they did in Fallout 3?