Tovald's Cave! It's pitch black in there except for the glowing mushroom's there's like 7 frostbite spider's 4 chaurus and like 100 falmer if you have Dawnguard installed as well there's also flying chaurus.

That can be a hard one. Any dungeon with lots of Falmer can be difficult, as well as Chaurus.

Forelhost or Skuldafn.

Too many full power Deathlords. And the Deathlord Archer with Ebony Arrows..ugh.

Oh, I forgot about Forelhost.