I've only ran into one so far... it was a frost snow yeti/troll thing on the mountain, on my way up to hrothgar. I actually had to set the game difficulty down two just to kill it, then set it back up. Otherwise it would just three shot butt-hurt me.

Probably giants. And dragons, but the first one I came across wasn't really difficult....and Spriggans. They are harder to me than the dragon!

Wolves made me jump several times, just 'cos they sneak up and then all of a sudden I get attacked

This. Drawing your sword then seeing a fireball coming your way and the loud explosion it makes... scary stuff. Wizards are TOUGH. I also doubted people saying magic is no good because they're the hardest NPCs in the game IMO.

giants. they will ruin you.

Frost Trolls- Not really afraid, just irritated.
Dragon Priests- But it's fun to say: "Yeah! I just got your face!"

Mud crabs, they always pop up and startle me.

Giants and 2H weapon enemies.

Some enemy casters can be a pain too because just about every spell they cast slows you. I also had a Storm Master or something like that cast a barrier and then an unending stream of lightning bolts. He wiped me pretty quick so I TGM'd it just to see how long he could maintain the barrier and cast lightning and after 5 minutes it was pretty obvious he had infinite mana. So I reloaded and ignored that dungeon for now. I can't recall where it was off hand.

I have a phobia of spiders IRL, so seeing the giant frostbite spiders on my large-ish tv at 3am isn't a pleasant sight. I slightly panic when I see them and can't look directly at them...

Frost trolls always annoy me so do saber tigers. But I avoid Giants all the time.

That one messed me up to, so I just used Adrenalin Rush and sprinted by it. Left Lydia to deal with it. Dont worry, she lived. Did the same thing when I went through Labrynthine, only there were 4 of them to get by. Lydia still live through that.