I particularly like vermin. The problem is coming up with an explanation why you might encounter different kinds of vermin in the same place.
For instance, recently I was scratching my head trying to figure out a reason why the PCs would be attacked by a giant praying mantis, a few centipedes, etc at the same time; that's not normal insect behavior!
I found one solution is to have various humanoid groups that use insects, either as mounts, symbiotes, or weapons. My PCs are coming across some jungle-dwelling cannibals that use praying mantids for mounts and guards, use immobilizing centipedes as grenade-like weapons (tossing them at the feet, letting them attack as normal), and use corpse slugs to compliment the ghouls they venerate.
The nice thing about vermin, too, is the variety and availability for inspiration. The animal kingdom is full of very curious species. Take for instance the wasp that shoves its stinger in its prey's brain, controls them and makes them walk into a hole it can lay eggs on them. That's a really
cool idea for a monster, mind-controlling wasps! They're also fairly simple, and easy to sprinkle into any environment or situation.
(Plug:
Blackdirge's 4e BUGS pdf; I had no part in making it, but I like and use the contents!)