Dangerous Denizens. It's always felt like a cool 'low level' monster book, in spite of the handful of high CR creatures, which is a good thing IMO. I've used it more than most of my other books, and it's mechanics are spot on.
My only complaints are the region maps, which are useless in my copy, since they look like gray splodges rather than maps, and the large number of silly golems.
I'm a fan of the Creature Collections as well, but the art & mechanics is a big blow against them.
Honourable mention goes to the WoW Monster Guide, which I'm reading at the moment and enjoying quite a bit- I'm surprised overall at the quality of the Warcraft stuff.