Henry
Autoexreginated
Up until the Monster Manual 3, I would have agreed with Abstraction. 
However, there's some reason that collection of monsters just "clicked" with me. Maybe it's the large amount of high level monsters in it (which is something that, even with the MM 3.5, is still unbalanced in that book). Instead it could be the unusual monsters (such as the Arcane Ooze or the Shadesteel Golem) in that book. For whatever reason, I couldn't see myself buying the MM2 or the Fiend Folio, because they didn't have but maybe five monsters each I'd ever use.
On the other hand, I could see myself buying Tome of Horrors (and I may yet, one day), because it fills those gaps that were left behind in the 2E3E transition.
As for "Monster, then scenario," or "scenario, then monster," I do both. Sometimes a whole scenario is born around a cool monster (MM3's astral stalker would be an adventure in itself!) Sometimes, I need something to fits a certain role (hmm, wizard's alchemy slough-sewers... what would be down there?) and that's where some of the weird beasties come in. Sure, I could (and do) modify existing monsters, but there's also that "what the


" effect that players get when they see a critter they've never seen before.
But the crux of it is: I could modify monsters instead of paying someone for their ideas. I could have had the very same ideas; however, I DIDN'T and they DID, and so I bought their book.
However, there's some reason that collection of monsters just "clicked" with me. Maybe it's the large amount of high level monsters in it (which is something that, even with the MM 3.5, is still unbalanced in that book). Instead it could be the unusual monsters (such as the Arcane Ooze or the Shadesteel Golem) in that book. For whatever reason, I couldn't see myself buying the MM2 or the Fiend Folio, because they didn't have but maybe five monsters each I'd ever use.
On the other hand, I could see myself buying Tome of Horrors (and I may yet, one day), because it fills those gaps that were left behind in the 2E3E transition.
As for "Monster, then scenario," or "scenario, then monster," I do both. Sometimes a whole scenario is born around a cool monster (MM3's astral stalker would be an adventure in itself!) Sometimes, I need something to fits a certain role (hmm, wizard's alchemy slough-sewers... what would be down there?) and that's where some of the weird beasties come in. Sure, I could (and do) modify existing monsters, but there's also that "what the
But the crux of it is: I could modify monsters instead of paying someone for their ideas. I could have had the very same ideas; however, I DIDN'T and they DID, and so I bought their book.