I am something of a monster manual extremist. For a generic game, which the D&D core set is, the 4E MM is absolutely what I want to see: Lots of crunch, lots of monsters, but empty vessels who I can fill up with the personalities of my own worlds.
For something that is setting specific, though, I am absolutely 100% the other way. My ideal setting specific book o' monsters is WHFRP's Old World Bestiary. It's the bible by which I judge other manuals that claim to be fluff rich.
Paizo's monster compilation is another example of their high quality creative work, but for me, who doesn't run pathfinder, it's a bit of a non starter. That's the double edged sword of a fluff heavy approach- great if it's the setting you want, worthless to anyone else.