Who needs a monster manual =D
I'm only half-kidding. I've been using and testing the general use monster stat blocks from Forge of Foes (
preview available here). It's amazing how easy it is to run monsters like these and players really don't know or really don't care. It's made prep even faster for me.
I'm not ready to throw out monster books completely but it's often easier for me to come up with a creature concept, pick an appropriate CR, use those stats, and improvise unique abilities or just flavor the monster with descriptions than it is to look up monsters.