Obsidian (or the Black Isle Umbrella)- I think this is a given by now considering I'm on the forum. But I've made it my life's mission to play every Obsidian release (Just bough Alpha Protocol today so I'm looking forward to starting that). I would say Inixille, but I've not played enough games by them to really judge. So for now, I'm going to kind of group them with Obsidian along with Black Isle Studios, which reminds me I should really play more BIS outside Fallout.
Kojima Productions- Kind of cheating as I've only really played the Metal Gear series from these guys. But considering Metal Gear is my favourite series, it feels criminal not putting Kojima's company on here.
Rockster/Bioware- Again, I'm cheating on this as I put two companies at a tie. I would say Rockstar for making open-world interesting. Most companies (Ubisoft) don't really know how Open World Works, Rockstar just make their games interesting. San Andreas is just a fun game overall and I adore GTAV.
Bioware, mostly because I like KOTOR, Neverwinter Nights and Mass Effect. When they get things right, they really get things right. While I admit ME3 has its flaws, I find a lot of them were down to EA. Otherwise, I genially enjoy their releases.
Honorable mentions go to
Square
Sega AM2
Project Soul (Pre Soulcalibur V)
ID Software (Pre Rage, I still need to play Doom 3 through)
Bungie (During the Halo days, but mostly because I had good memories playing Halo)