In no order:
Black Isle Studios (RIP) - Classic RPGs, Planescape Torment alone would put em in the top 10, Fallout would push em into the top 3.
Looking Glass Studios (RIP)/Irrational - System Shock + Thief games
Troika (RIP) - Leftovers of Black Isle, Arcanum + Vampire Bloodlines were fantastic games with massive bugs.
Obsidian - Rest of Black Isle's remains, great concepts terrible execution but I love em for trying.
Valve - Of course
Blizzard - For selling out but not going to shit
Bioware - For Mass Effect series alone, nevermind NWN & BG & IWD etc etc etc
Relic - For doing what they do best better than almost anyone - RTS games. See COH for crowning achievement.
Microprose (RIP) - Civilization 2 might be the best game ever made.
Firaxis - No no wait Alpha Centauri IS the best game ever made. WHERE IS MY AC2 SID WHERE IS IT YOU BASTARD!!!!