Someone that's going to get alot of the votes:
I gotta go with Monte.
Why? There's often a tradeoff in d20 products between creativity and sound mechanics. Monte tends to avoid this, providing both at once.
The balance of his products is usually at or just slightly above what I try to maintain in my games (I did not like the Lace Spell feats, for instance).
Products that I particularly enjoy:
1) The eldritch might series. This is Monte at his best.
2) The DMG, the best written of the 3 core books.
3) Most importantly: Arcana Unearthed
Oddly enough, I didn't like what I saw of the Book of Vile Darkness. Admittedl I have not perused it more than casually, but just giving a +2 profane bonus to this or that hardly seems like the originality that Monte usually manages to produce. I'd be happy to be shown other content of the book that proves me wrong, though

(Note that I'm not saying or hinting that anyone should post it here; that's vile [ba-dum ching]. I'm talking about IRL).