A few, off the top of my head (certainly not a complete list by any stretch):
Grasshopper - Their games are generally rough around the edges, but they're actually inventive and interesting, which is far too rare in the industry.
From Software - They've created some of my favorite games and series, and they're pretty consistent in quality.
Intelligent Systems - I hope these guys never stop making great SRPGs.
Love-de-Lic - I can only hope I'll be able to play and enjoy every game they made someday. I list them as a favorite developer since the studios that resulted from its closing have made some of my favorite games.
Valve - I would love them forever just for Half-Life 1.
Sonic Team - Hahaha, just kidding.
And since you didn't specify if you meant development studios or individual game developers, I'll add:
Warren Spector - In addition to being at the center of some of my favorite games, he seems like a genuinely great guy.
Goichi Suda - He's one of the only developers that gives me reason to believe at least some games will interest me in 5-10 years.
Shigeru Miyamoto - Though of course some people overestimate the importance of his role in the creation of great Nintendo games, I still think it's apparent the man knows what he's doing when it comes to game design. I don't think much else needs to be said.
Tetsuya Nomura - Hahaha, just kidding.