Sierra and Free Radical would both be on my short list; both gave me many hundreds of hours of childhood joy. I'd also add Sabarasa Studios to the list. They only managed to get one game released, with an updated remake ready for release, before the logistics of getting games published internationally as a small South American developer pulled them under. It's a shame, because their one game, Mazes of Fate, was an excellent little dungeon crawler in the vein of Dungeon Hack and the classic Eye of the Beholder trilogy.