Good monsters

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.

Yes, I think going with a more simple monster is the best way to start. You can always add more strange ones later, but for the first mystery a simple bad guy (maybe with some minions) means the hunters have time to get to know each other rather than spending all the time working out what's going on. It also means you, as Keeper, have a bit less to keep track of while you're learning how it fits together.