I'm about to run my first Monster of the week game and I'm wondering what makes for a good monster.
Is it better to have a humanoid monster or is it possible to use more environmental threats?

My initial idea was to use a swarm of enhanced critters, which would make for a very passive threat, only reacting once players or NPCs stumble upon its lair; it would then go berserk to destroy all possible threats, finally resulting in the destruction of a nearby town.
However, thinking it over, I'm leaning towards using a directing intelligence in the form of a single boss monster, which would probably be easier to understand and more in line with the typical Buffy and Supernatural monsters.