These are the ones I remember off the top of my head.
FASA (RIP) mainly cause they made some of the best mechwarrior games (I don't like the new ones that much)
Bioware: Neverwinter Nights kicked arse! and then the followed it up with Knights of the old Republic! Then there was BG series that we all know and love ^^
Id: duuuuuh
Valve: Half life and Half life 2 damnit!!! two of the best shooters ever made.
Blizzard: World of Warcraft and Warcraft.
Dice: battlefield 2 yeeeehaaw!
Rockstar: Pissing everyone off everywhere they go with style!
Square Enix : goes without saying
Bungie: I give em props for almost making Halo an rts, but thats about as far as it goes, The FPS shooters really aren't that great, They won't stand time that well. (yeah yell at me)
westwood: (RIP)