My Favorites (In No Particular Order):
Wizards - Contrary to a perceived popular belief, I tend to like most of what they put out, even if all of it isn't perfect. What they publish is what I want.
Necromancer Games - I only own their ToH's line, but I and III are really solid monster books, though II's kinda weak, still good though.
Privateer Press - Their rules may not all be solid, but they've created some beautiful books and a great campaign setting. Now where's my
Monsternomicon II?
Honorable Mention:
Green Ronin - I only own a few GR books, but they've yet to really wow me with anything.
Advanced Bestiary is good, but it's such a hodge-podge of 'Holy crap, awesome!' and 'Why did this get put in here?' that I'm not sure what to make of it.
Book of Fiends is okay too, nothing spectacular. Still, overall good stuff.