Valve followed closely by Irrational. Both studios produce highly imaginative games that, while go through very long development processes, never disappoint. I don't think Valve has ever produced a game that has truly been bad and the ideas that come out of Irrational are mind blowing. When Episode 3/Half Life 3 comes out, it's going to be head-explodingly awesome. BioShock: Infinite is already on top of my must have list and looks to be truly mind bending. I love the creativity that these studios exhibit.