That is a very tough question. There are a large number of developers that I care for but I would have to say that ArenaNet is the developer that is my favorite at this time.
Other developers I really like are Camelot for giving my the Golden Sun series, Sucker Punch for the Sly Cooper series, SquareSoft for Kingdom Hearts 1, SquareEnix for Dragon Quest 8, Pandemic for Star Wars Battlefront 2, GameFreak for raising me on Pokemon, Nintendo for Mario Kart, SSB and the Gameboy versions of the Zelda games, Team ICO for Shadow of the Colossus, Gearbox for Borderlands (and hopefully Borderlands 2), Black Isle for Baldur's Gate: Dark Alliance 1 and 2, Intelligent Systems for Fire Emblem and Rockstar for Grand Theft Autos 3 and 4.
However, I've been getting into indie games recently (who hasn't) and Adhesive Games, MinMax, Taleworlds and Arrowhead are all awesome studios.