Hard to order these, so this'll be random.
Sierra - grew up with these games and played almost every one of them. The Williamses were kings of the adventure genre. Hell, even the FMV Phantasmagoria was pretty good. A true gem was Conquest of the Longbow, probably the best told and interesting version of Robin Hood you'll ever play or see.
Valve - While John Carmack is a technical genius and can create a great engine, these people brought some depth, AI and graphics to a genre that really wasn't putting out anything new at the time. They still are innovating using techniques that others haven't thought of to make games more like a movie or lifelike.
Interplay - I still have a copy of Star Trek: Judgment Rites. Of course the Fallout games, and a ton others I'm probably missing.
Westwood - created the C&C games, Kyrandia series and pretty much invented the RTS game.
Microprose/Firaxis (i.e. Sid Meier and crew) - All his games and even the remakes have been excellent. Civilization, Alpha Centauri, Pirates (old and new) all are addicting and fun as hell.
Honorable mentions:
Origin - The Ultimas and Crusader series were great fun.
Bullfrog - They made Sydnicate, still one of my favorite games of all time.
