I don't think I've seen sierra mentioned unless they're just a publishing enterprise?
What about Bethesda for Morrowind and Oblivion?
I would also put up Infocom - they were true pioneers in the early days of PC gaming.
I would've put them in my list, but they would have to make more than 2 great games (1 great game in my book FO3), the quality is their just not the quantity.
And EA who have been running for far longer then any other games company that i know of.
Forgot about Beyond Software - C64 stuff - Lords of Midnight, Doomdark's Revenge Quake Minus 1. All really advanced for the time, fantastic strategy.
I will always have a soft spot for Magnetic Scrolls. Guild of Theives was a classic for me.