Monster Vault tend to have the right math and also interesting mechanics.
Using dragons as an example. MM1 dragons tend to do little damage, drag the encounter a bit too long, and die to control effects.
MV dragons with their instinct actions solve most of the above problems, I think. But then MV dragons do not come in all colors. So if I'm to run a 4E game, I would grab the interesting individual powers from MM1, but use MV as a guideline about how to actually assemble the powers into a monster that can fit in its role.