Regenerators, YES. Holy shit. I don't know what was worse, them or the area in which you first encounter them. That old prison bit, when you start by hearing nothing and then you hear the groaning, bloody hell it's scary.
The Chainsaw Dudes were also a pain in the ass, even more so than the Bella Sisters. With the Bella Sisters, they were the focal point of the whole thing, you were constantly aware of their position, but the Chainsaw Dudes are just like, MFOAWMNAOMNG. I remember the first time I walked out of that house where you first encounter one, running back into the house and hiding in a corner. It failed.
THEN you have the Caesar's Legion from New Vegas. If you want badass, THEY are the badass ones. They enslave, crucify, torture, molest, rape, pillage. Yeah, they basically fucking own you.