Scary: Gargoyles. I hate that they pop out. I don't care how super awesome they look. I just really hate that they pop out and there's nothing I can do to sneak up on them!
Repulsive: Falmer. It's really a tie between them and Chaurus, but I chose the Falmer. If I were to encounter one in real life, I would feel so horrified by its blindness and clammy, white skin that I'd feel compelled to murder it to put it out of its misery.
On my first playthrough the first Falmer I encountered jumped from those holes they have right in front of me. It didn't detect me but was so close that it's face filled all my screen. I saw all the little details in it and it was horrible! "It's probably very dangerous" I thought and killed it with sneak attack. Then I learned more about their sad history and pitied them. Now I mostly try to sneak past them, leaving them to their business...
Esbern was cool when you first met him. I just wish he didn't get all huffy about Paarthanax and being a Blade. I liked it much better when he was the old, scatter-brained "loremaster" of the Blades. :) I wished I could take him with me so he could tell me all about the various ruins I was exploring.
For me there is nothing more frightening than going to a deep underground cave full of falmers, chaurus reaper and chaurus hunter specially if i play on expert or master difficulty.
I always remember my first encounter with them in Frostflow Lighthouse as i entered the Lighthouse i heard that strange noise then i saw the dead bodies and read their journal... my imagination did the rest... and then further deep in the bowel of that cursed Lighthouse i saw my first Chaurus and they fulfilled my expectations of the repulsive creature mentioned in the journals. Well done Bethesda!