Regenerators from RE4 should definitely be on this list. Not only do they take tons of damage and regrow their limbs, but after you've blown off their legs and feel like laughing at them for not being able to move, they slither MUCH faster than they were walking and jump at your neck. They also have a low, ragged breathing noise that if you don't hear early on they'll jump you. Jesus Christ I hate them.
The scariest villains EVER