Hmm... currently, only 2 really: Nintendo and Atlus. If NIS America can get their editing act together, then I'll consider adding them to the list. (Sorry, but if I buy a US game, I don't expect to have to read "クロチェ様." I can, but that's not the point.) If I'm buying older games, then Enix, and Square if it's pre PS1. (Square-Enix is not the same companies anymore.) I'd also be interested in seeing Broderbund come back to consoles, since I did enjoy a few of their NES titles...

Pretty much any studio that consistently puts out good RPGs.

Atlus, Nippon Ichi, Level 5, Bioware, tri-Ace are almost always a guaranteed day 1 purchase.

Square-Enix used to be a must-buy but I've been disappointed by some recent titles and will likely find myself waiting for cheaper copies of a few upcoming games.

It's pretty easy to tell from my collection. I like Nintendo games and Sony games and many of Microsoft's games, I like Bungie and Squaresoft and Obsidian and Bioware and Treasure and Valve... all of those are reflexive. I support the studios by giving them my money.

The one I support, though, that I don't think anyone else is going to say?

Silicon Knights. because I know they have the capability to make something truly breathtaking.

This sounds fair. Incognito, Level 5, Team Ico, Valve, Polyphony Digital, and Capcom all have good track records so I'm not skeptical with their releases as I am with some others.

Sony first and second parties
Kojima

nintendo and square enix

Sony Santa Monica
Insomniac Games
Naughty Dog
Kojima Productions
Team ICO
Media Molecule
Criterion games

I always support whoever makes good games I'm interested in. Typically Nintendo gets the most support from me, but recently 3rd parties are starting to catch my attention.

Naughty Dog
Valve
Media Molecule
Kojima Productions
Remedy

I would post reasons, but I'm in a rush to leave.

Not really supporting a studio but certain game franchises I always buy.
Zelda including hardware purchase. (please not another boy becomes green clothed hero and gets the master sword story. MM was great because it was different.)
FF
MGS
These developers while great dont make me support them just certain franchises. Although I do like Capcom, they seem to try to support all formats.