First I'd say Paradigm Concepts. It's not big yet, but they're visionary. The stories, the political interplay, the chances for honest-to-gosh heroism, that's all I got into RPGs for in the first place. They're my number one, because every product they put out gives me an idea for fifty different characters and twenty different campaigns. On second thought, I hate them. I'll never in a lifetime be able to explore all the ideas and hooks in their products. Hate hate hate.
White Wolf definitely. Yeah, their Sword and Sorcery stuff is broken as heck, but it's fun. And they're bringing back Ravenloft, and so I've heard new Masque of the Red Death rules. Victorian horror is just fun.
AEG, just because they're AEG. Rokugan has an incredibly hard-core following, and Spycraft fills a niche in gaming that even traditional gunrunner games, like Shadowrun and such, doesn't fill. They do great stuff.
Fantasy Flight Games might belong here. Their stuff is eclectic, I only really got into one of their campaign settings and they're no longer supporting it (I wish they'd reworked Dragonstar instead of dropping it, it was a great idea, just needed some mechanics tweaking.) But their splat books and suppliments are great stuff, and a boon to any DM.
And that one company . . . I think Avalanche? I think? Just because you have to admire a company who goes straight to the lowest common denominator of our beloved hobby and puts a ridiculously and anatomically impossibly well-endowed woman on the cover of every single product they make. I admire someone who makes no pretense of being civilized like that. They made a decent book on Dracula too. And strangely enough, most of the cover of that one, if I recall, was taken up by a very busty rendition of one of Dracula's vampiric ladies . . . ah well.