I'll start.
Withered Foxy - He takes marginally longer to deal with than most other immidiate threats, plus his sound effects are pretty loud.
Endoskeleton - They pair awfully with any character that uses the flashlight, such as Shadow Bonnie and Shadow Freddy.
Mangle - I haven't memorized every camera's corresponding number so it's hard with the puppet and any other camera threat, plus their kill timer is crazy short.
Malhare - Having a set limit on cam flips just makes me feel paranoid.
Plushtrap - I think if you flash the wrong direction he kills you immediately, which sucks when paired with Shadow Bonnie or Nightmare Bonnie in a randomizer. He's also generally hard to keep track of in my opinion.
Nightmarionne - Similar to Withered Foxy, they make a lot of noise so it's hard to listen for other threats, but can also spell death if Dreadbear or Toy Chica pan to the same side.
Ballora - Honesty... I never learned her mechanic properly. I got her with Lolzhax then just deathcoined her immediately.
But that's just my opinion. And yours?