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It's really hard for me to narrow it down to just one.
Forgetting the core MM, I have to go with Tome of Horrors (I). Close on it's heals, however, is Monsternomicon.
As favorite monster books, well except for the obligitory MM. I'd go with the Monsternomicon first, the beasties there rock, and the Book of Fiends gets a good nod by me.
Monsternomicon all the way for me. Although the Revised Creature Collection 1 and Denizens of Avadnu come close. I wasn't overly impressed with Tome of Horrors 1. It felt like nothing but a collection of stats with no fluff to beef it up. Good thing I'm an old-time player and know some of the background on many of the monsters in those pages, otherwise, it's utility would have dropped greatly.
Some other great monster books:
Creatures of Rokugan
Bestiary of Krynn
Monsters of the Mind
Liber Bestarius
Penumbra Fantasy Bestiary
I went through Tome of Horrors I with a pad of post-its to mark things I want to use. I would have been better off putting post-its on things I *didn't* want to use.
Other than the 3.5 MM, Monsternomicon is by far the most fun to read, although I curse it for not having a chart that organizes monster by CR! *shakes fist*
Other than that, Draconomicon, Tome of Horrors I, MMII, and then Fiend Folio.