Black Isle and now Obsidian. Sure, their stuff is often buggy, and doesn't have the most amazing graphics but their stories are well made and very interactive, plus their gameplay mechanics are always very well thought out.
I would say the same of Troika, for the few games they've put out. Oh, and Generally Rockstar has my loyalty as well, they've let me down rarely enough that I assume the Rockstar logo means it will be fun. Oh, and Bioware, but I feel they get last mention as they are my least favorite out of this whole collection, their stuff is great, it's just... what I said earlier about Obsidian's games always featuring really well thought out, if buggy, gameplay mechanics, well I can't say that about Bioware, sometimes they get lazy or maybe just not creative enough, and their stuff is just as buggy... Still though, I love The Old Republic and pretty much all the Bioware games (though Dragon Age 2 made me want to cry, I don't want a Midevil Mass Effect, I want more Dragon Age).