Stuff you can't go wrong with:
Green Ronin- probably the best of the best right now, I haven't seen a single GR book since they started that was a stinker. Top notch stuff. Especially check out the Shamans Handbook, Witch's Handbook, Book of Fiends, Unholy Warrior's Handbook, and Book of the Righteous.
Mystic Eye Games- Again, some great stuff, from their Foul Locales series of min-adventures, to Artificer's Handbook, Guilds and Adventurers, etc. MEG stuff tends to have a darker feel though (which I love), so be warned.
Atlas Games- Their Penumbra line is incredible, but is more similar to medieval earth than really out there fantasy. Their adventures are some of the best, and can't be solved by hack-n-slash. I love it, but some folks think its too grounded in reality.
FFG- Another golden company, their whole Legends and Lairs series of books see tremendous use at my table, and they brought us Midnight and Dawnforge!
Necromancer Games- These guys consistently put out some of the best adventures offered, with a very "old school" feel. Not for the weak at heart! Tome of Horrors kicks butt too.
Paradigm- Probably some of the best flavor text books out there with solid mechanics, and makers of the much underappreciated Arcanis setting. Thier book on Mind Flayers was phenomenal (Unveiled Masters), as was their giants book (Lord of the Peaks). Forged in Magic (a magic item and spell book) is good too.
Kenzer- I personally like Kenzer's stuff a lot. Kalamar is a top-notch setting, although some people find it boring because it is very well researched so it makes sense culturally, geographically, and politically. Their rules tend to be pretty solid, and the Kalamars Player's Guide gets regular use in my game.
Bad Axe Games- The racial books they have put out have been great, but I have only one complaint- I want them to be longer!!!!
Companies on the border:
AEG- These guys have some great ideas, but I've had a few problems with some of their mechanics which seem overpowered (especially Feats). Still, most of their books are packed with great plot hooks, optional variants, and good ideas- they just might take a little tweaking. I especially like Wilds, Evil, Good, Mercenaries, and Toolbox.
Malhavoc- I know lots of people love Monte, but I've gotten very little use out of any Malhovoc purchase. The rules are overall pretty good (mild to moderate power inflation), but many of his ideas and tastes are just too wonky for me. How many times in a game will I have gods dying, meteors striking the planet, or need super-specialized prestige classes that give up almost nothing while gaining tons of powers? Anger of Angels was pretty good, but then again, how often do you use angels in a game, much less as PCs?

The Books of Eldrich Might have some neat spells and feats, but many have proven to be overpowered in my game.
Troll Lord- These guys have some pretty decent adventures, but you'll have to double check some of the stats to make sure they are ok.
Bastion- If I liked really high fantasy, I'd like this company better. They have some out there stuff, but it tends to be well done. The big complaint I have about them is that their books are EXTREMELY pricy for the size ($25 for 96 pages softbound).
Mongoose- When Mongoose is good, they are really good, and when they are bad, they put out some stinkers. The Quintessential series is hit or miss (I liked Revised Fighter, Rogue, Sorcerer, Witch, Barbarian, and Paladin), as well as Book of Dragons. The Slayer's guides are optional, but have some decent background infor on a lot of critters. I'd say the Conan RPG is the pinnacle of their achievements so far- its golden. The Ultimate series hardcovers are ok to good, with Ultimate Equipment Guide and Ultimate Arcane Spellbook being the best. That said, even the worst of the Mongoose books still have some gold nuggets in them I use.
Companies I'd avoid:
Fast Forward Entertainment- Widely known as probably the sloppiest publisher, their stuff tends to be super overpowered and the mechanics tend to be rather shoddy. That said, two of their recent products have been pretty good (Devil Player's Guide and Deadliest Creatures Tome).
Sword & Sorcery- Moderate to good production values, but the mechanics on a lot of their stuff stinks to high heaven! The bindings on almost all their hardbounds are crappy too. Their stuff is extremely overpowered, and I don't use Relics & Rituals at all anymore except for Ritual magic. What isn't overpowered is so Scarred Lands specific to be of very limited use in any other world. Be warned!