I'd have to say Rockstar and Blizzard. Both seem to be of the philosophy they will take their time to make things right and it shows. Some releases are better than others but I can't think of any by either one that was at least a 7.5 or 8.
Harmonix. Yes, they're a niche company but they don't half ass anything and even tho they're an over-saturated game market they constantly get good reviews, unlike a certain other music game company :-P
For major companies, I gotta second Rockstar for sure.
Treasure's got plenty of missteps. They just pretend they never made the games that are missteps (Dragon Drive, Advance Guardian Heroes, Stretch Panic, Bangaioh DS. Some people consider PS2 Silpheed a misstep too, but I don't.)
I'm going to nominate PlatinumGames (FKA Clover) on this one.
Nintendo did license all those horrible educational and CD-i games, even if they may have had little input on it.
Bethesda has had bad games, like the fairly cheaply made IHRA drag racing games and the Home Alone game. They also had a lot of Terminator franchise games, but I haven't really heard anything about those.
Naughty dog had some early flops (Way of the Warrior, Rings of Power) as well as Jak Combat Racing. It also had Uncharted 1 which, frankly, I didn't think was that good overall.
Sucker Punch has a really good track record, although I think inFamous could have been so much better than it was.
Rockstar has had some less than stellar games. Earthworm Jim 3D was really, well, unspectacular (and their only Earthworm Jim game). They also had some cheap Austin Powers games. However, as of late, they really have been on fire.
I would nominate Team Ico, but since they only have 2 games, that seems like cheating.