The Morning Suit Man said:
Valve, because Valve.
Also because they technically publish everything on Steam. So yeah.
Steam is a platform for delivering digital content, and doesn't so much displace/replace other publishers as give developers a direct line to a market/existing consumer base. And Valve has grown more and more developer friendly of late. So definitely yes, thank Gabe for Valve.
Also on my A list are 2K Australia, Ubisoft Montreal for FC2 and FC3,
Paradox not just for a host of good strategy games, but for allowing/encouraging modders in the Paradox forums. You really can't take that for granted these days. Bonus points for the Europa Universalis III "Raaaaage!!!" thread. It's made of internet memes but it smells like win.
Bethesda gets points for feeding iD Games and the Fallout/Elder Scrolls franchises; but points off for Quake 4, Rage, and Brink. Yeah, yeah, these each had a reason for being, but could have been better with relatively little effort, and in Brink's case someone needed to say that it doesn't matter what exclusive clothes the Emperor is wearing, he's still walking behind a manure spreader. Runner up.
Kalypso would rate higher if they had slapped Haemimont around until they made Omerta a better game. I'd gripe about DLC, but many of the Tropico add-ons are reasonably good. Honorable mention.