Top 3, in order...
EPIC Games: Despite some early bumps in the road, Unreal Engine 3 has really become the ubiquitous go-to engine for developers to license. It's pretty much everywhere now, and has more than taken over the position that Renderware held last cycle. Additionally Epic entered into a sweetheart publishing deal with Microsoft that's resulted in the gritty and acclaimed Gears of War, far and away the best selling new IP on the HD platforms this generation. They've really become a powerhouse in both tools/support and the strength of their own games in recent years.
Level-5: After hitting the big time with Dragon Quest co-development in 2004, L5 smartly reinvested that capital with an eye at becoming publisher themselves. By going against the grain and by focusing on new ideas that make sense to modern game audiences, they've already managed that with the worldwide success brain-puzzling Professor Layton, and have other surefire hits in the wings like the kids soccer-RPG Inazuma Eleven, the Gundam meets Poke-clone Danboard Senki and the high profile collaboration with the renowned Studio Ghibli, Nunokuni. They're one of the unique success stories coming out of Japan these days, and even more surprising they've managed it entirely on the back of new brands and new markets.
Valve Software: Having cultivated Steam as an empire (and acknowledged platform) unto itself, Valve been on the forefront of also being a PC developer cultivating a new dedicated console audience. The Orange Box first really brought unparalleled value to gamers this gen (not to mention the most innovating fps/puzzler in recent memory, Portal) and Left4Dead was the sleeper hit of last year and has grown into a AAA series itself already. Valve seems to be be making all the right moves today, both at home on PC and branching into a new equally dedicated 360 audience.
...honorable mentions go out to Blizzard Entertainment, Bethesda Softworks, Capcom, Ubisoft, Bioware, Infinity Ward, Hudson Soft, Armor Project, 2D Boy and thatgamecompany.
