Shelob!!
Ordinary sized non-poisonous (UK) spiders are bad enough, even when they are 100 times smaller than you. Now a Giant spider much bigger than you, with a pitch black lair and uncuttable sticky webs - er nasty!!! Not to mention the foul smells and orcs round the corner.
I suppose Ungoliant too but (thankfully) we don't get to see her close up,
brrrrrrr!!!!!!!!