I'm not a publisher, just the average Joe Consumer.
WotC - Toss up after the core products... MM2 is good, especially if you need higher level monsters. If you're looking for a setting, I'd really recommend the FRCS and some of it's campaign books like Magic of Faerun and Faiths and Pantheons.
Sword and Sorcery - Relics and Rituals 1&2 should be on every D&Der's bookshelf.
AEG - Magic presents some very neat ideas, Mercenaries is a general book about combatants presenting new races, core classes, prestige classes and the like. Worth looking into.
Mongoose - I'd look into their Ultimate line of books. (e.g. Ultimate Prestige Classes, Ultimate Feats) - they serve as a compilation of published material from Mongoose as well as other publishers.
Privateer Press - Monsternomicon is the only product of theirs I own, but it's quality can't be beat. The monsters described are for their Steampunk-style setting but oh so adaptable to "normal" fantasy role-play.
Bastion Press - Oathbound gets lots of loving from almost all who own it. I do not but it looks pretty interesting.
Necromancer Press - The only product of theirs I currently own is Necropolis, which looks to be a good Egyptian style adventure and sourcebook. Another product that is getting a lot of attention is Tome of Horrors, which updates some 300 ODD, and 1e/2e monsters into the 3e system. I believe it also has 100 new monsters as well.
Ambient Press / Mystic Eye Games - Ambient is perhaps best known for their Prestige Class books, but their best work IMO is released in print by MEG under the name "Necromancer's Legacy"
Atlas - I only own two of their products, Occult Lore (excellent) and Nyambe (great African setting)
Speaking of Mystic Eye Games, look for a product called Bluffside if you are looking to run a city campaign. (You may want to look for it under the Thunderhead Games product line)
Fantasy Flight Games (not to be confused with Fast Forward Games) - I would say the Path Of series to be the most useful of theirs (I own Path of the Sword and Path of Magic)
Green Ronin - they put out a plethora of good products, but IMO, their best product, indeed the single most useful d20 supplement I own is The Book of the Righteous. I would strongly suggest that any DM give this book a read. I don't think you'll be disappointed.