List them in any way you choose! Rather than pick an absolute favorite, I thought I'd put them in tiers:

Definitive Favorites:

Valve

Blizzard

Nintendo

Second tier:

Epic

Maxis

Ico

Third tier:

Id

Firaxis

Keep in mind that my "third tier" are still in my favorites -- just a couple notches below Valve etc. I'd put them above every developer not mentioned here. Anyone else?

what about square enix, Bioware, or Ubisoft Montreal?

 They aren't on my list. If those are on yours, then show your own list. That's my point :p

I haven't enjoyed an FF since VIII, and haven't loved one since VII.  

Godly - Polyphony Digital Noughty Dog Kojima Insomniacs SquareEnix Sony Santa Monica Team Ico Konami Factor5 THQ

I wish there was a way to break these up a bit more. For example I don't like all of Ubisoft but I love the group that did Beyond Good and Evil. =/

Anyway here are my Favorite Developers!

Definitive Favorites:
- SquareEnix
- Nintendo
- Retro Studios
- Team Ninja
- SNK Playmore
- Nippon Ichi Software


Second tier:
- Intelligent Systems
- Dimps
- Tecmo
- HAL Laboratory
- Camelot Software
- Bungie
- Ntreev Soft

Third tier:
- Ubisoft
- Capcom
- Crystal Dynamics
- Amusement Vision
- Epic Games
- Blizzard Entertainment
- Naughty Dog

Nintendo Sakurai Game Freak Blizzard Retro Studios Square Enix Capcom Ubisoft Sega (yeah, I know, most of them suck but some are still great) Blue3, are you citing Factor5 on the basis of a game you have actually never played (Lair)? Just curious lol.

Yeah me too CapAmerica. I had to put Ubisoft on there for Beyond Good and Evil, even if they never make another game I like lol

1st Tier Rareware ( for the n64 games and for SFA, loved it) Nintendo( Anything they create is good or excellent) Intelligent Systems ( 2 of the funniest franchises ever and my personal favorite, Fire Emblem) 2nd Tier Square Capcom ( for RE4) Namco ( Tales ) Retro 3rd Too many

Nintendo Bioware "Old" Rare Silicon Knights Lucasarts

Nintendo, Epic, and Bioware. Easily the top three.