What are the mooks, minions, and fodder in your games? The enemies that your party can mow down in large numbers, guilt-free? Does your game feature a lot of mindless robots, undead, or some kind of evil empire full of faceless enemies to tear through?
I like to use zombies. Tons of zombies. They're a fucking pain for people to deal with, especially in large groups.
It can be overcome with a little action economy, but still, they're difficult.
And I like goblins and bandits. Or goblin bandits. You can't go wrong with the classics, and sometimes you get an interesting RP experience out of it.