My favorite monster manuals:
Although I am fond of the 1st edition Fiend Folio, 

 

my all-time favorites are the Hacklopedia of Beasts series put out for Hackmaster IV by Kenzerco. The covers alone are worth getting the set for, but I also love the depictions of the monsters inside where most of them are demonstrating their hilarious lethality against hapless adventurers. It really captures the danger of Old School RPGs for me. Also, Hackmaster IV being a parody, the books contain a weath of humourus monsters like the Dire Armadillo, the Attention Getter, the Speckled and the Spoted Arvenger, the Augerlisk, the Bandy Tailed Fighting Cock, teh Great Red Marauding Beaver, the Carrion or Dire Camel, the Grunge Elf, the Hedley Kow, the Latrine Ambusher, the various Monkeys - Blind, Flying, Ice, Musk, Poison and Six-Ribbed Skink. The War Moose, the Fetid Mummy, Samaurai Jackals, Carnivorous Sheep, the Shirkmare, Simian Orcs, the Slapping Durkin, the Slobgoblin and legions more.

The other great thing about the HOBs are the Yield sections in each monster description that lists the uses of the creature's various parts, including what is useful for spell components. I've used the creatures in the HOBs for a wide variety of games including D&D Gamma World, Rifts, Traveller Star Wars and Hawkmoon. I have found the HOBs to be a very useful tool as well as an amusing read and I would recommend them highly to any GM.