I like Ubisoft, not saying they're my favourite but since Valve and Double Fine have been mentioned, I'm putting up Ubisoft here.
Now, granted they don't always hit the game on the head properly and then kill it with sequels, the way they kill it with sequels is because their game isn't perfect. That's the best approach to it imo, Prince of Persia SoT had crap combat, so the sequel fixed that, but then the platforming was gone. So in the third one they fixed both but killed the narrative somewhat. The same happened to Assassin's Creed, the first one has faults, they were revised in 2, revised further in Brotherhood etc etc. Yes they milk the cow a bit but that's not really the developers in my opinion, that's the producers who will do anything for a quick buck (in ANY company). I quite liked the Farcry series too and Tom Clancy's series has grown on me somewhat. Oh, and I grew up with Rayman...THE OLD AWESOME RAYMAN, before stupid rabbits invaded the world ¬_¬
My runner up of not being mentioned properly will be Frictional games. Sure, they stick to the same genre but they put so much heart into it, it's done oh so well. They're an indie company also, so asking so much from them would just be cruel.