This was a tough one. When I think about this question I have four that immediately come to mind. They are Wizards of the Coast, Green Ronin Publishing, Goodman Games, and Paizo. They all make my favorite games.
Ultimately I have to go with Green Ronin because they publish Freeport, Mutants and Masterminds, and the AGE system. In addition, there 3rd Era books were always the coolest.