The most? At this time, game-play is in the heart of an African-esque continent, so I've used a lot of critters from Nyambe, which isn't a critter book but it does have a decent critter section.
Second to that being races with Class/Level advancement drawn from various sources...
monster manual gets used more then all the others combined, but I do get yuse out of MM2, FF, Monsternomicon, Mopnsters of Endless Night, and Book of Fiends.
I used to get use out of the Tome of Horrors but I gave away both my copies....