The best one imo:
Microprose: Brought us civilization, transport tycoon, rollercoaster tycoon, railroad tycoon and Star trek: Birth of the Federation.
Then in no particular order:
Valve ( speaks for itself)
Rockstar (north) ( GTA, and erm, GTA

)
Westwood ( C&C and RA series, speaks for itself)
Ensemble studio's ( Microsoft) ( I was blown away by age of empires initially, and enjoyed AOE2 extensively for years and years, played in a clan for ages too.)
Maxis ( sim city games are superb, and let's be honest, the first sims was decent and enjoyable)
ID soft ( Quake 3 engine brought so many good games inc. Mohaa, CoD, Wolvenstein, etc., also commander keen 4 was the first ''proper'' ( eg. not tetris/paratrooper/frogger) game I had ).