#1 Fantasy Flight Games- the Path of ... books are the BEST classbooks out there, and all of their Legends & Lairs stuff is what the industry should shoot for (take note WotC). Midnight is phenomenal- hands down the best campaign world out there. And the recent release of Redline has me itching to run a post-apocalytic game ala Fallout.
#2 Green Ronin- Consistently great stuff, very adaptable, never a stinker. Legions of Hell, Armies of the Abyss, Book of the Righteous, Hammer & Helm, Testament, and Unholy Warrior's Handbook are masterpieces.
#2.5- Mystic Eye Games- Great stuff, I love the Foul Locales series, and its suitably dark and gritty for my tastes. Also, one of the more innovative and unconventional publishers out there.
#3- Mongoose- When they are good, they are REALLY good, and rarely they aren't so good.
#4- Necromancer Games- I'd rank them higher, but I usually write my own adventures. That said, the Tome of Horrors is great, and the adventures I do have and have run by them have been incredible- the best adventures out there for d20. They are very adaptable, and easily accomadate extra DM plot hooks and RP opportunities.