As my high levle mage, starting with wimpiest:
Anything not below: Pathetic

Bears: Low normally, but i once got killed by a single bear a minute after downing an elder dragon and a frost dragon at the same time

Dragons: Low (Lightning Storm pwns them, although if they bite me they will one-shot me)

Ascendant or above [caster type]: Moderate-Low, I can out-spell them but sometimes they heal and a group means i have to dodge a lot

Dreugr with Bows (only those with bows): Extreme. They outrange me really, and do what i estimate to be 200-300 damage per shot Base Damage on Adept (more than any of my spells can do). If they shoot first i lose. If i shoot first but miss, they shout me, then shoot me, i lose. Only way to win is shoot first, hit, and stunlock them. A dreudr scourge or deathlord with a bow is the only enemy type that can kill me without lots of luck or numbers.

Those high level vampires outdoors near Solitude. Read about them in a book...ran into one. First challenge I had in a while.

FFrost trolls are nothing to this high level mage. Fire, fire and more fire. Or, more precisely, flame cloak, summon dremora lord, double Incinerate, fire breath. I took out a pack of 5 in seconds

Hardest tend to be those randon vamp or mages with Ancient or Master tag on them.

Yeah, in arcwind pass I actually saw a dragon fighting a draugr deathlord.
The deathlord just mangled his supposed 'master'

dragur deathlords and volkihar vampires are harder to kill than dragons because you can face 2 or more than a time unlike dragons I was in a dungeon and went against 3 melee deathlords and 2 archer deathlords it was my most overpowered character and he still had trouble

The two most difficult enemies I find are:
Draugr deathlords (ebony bow + ebony arrow), they hit for 300 damage on master if you have full physical resistance.
Forsworn briarhearts, dual wielding bastards, their dual power attack will kill anyone if you don't have [censored] loads of health.

I'd say the briarhearts are the worst though, draugr deathlords go down pretty well with a few incinerates on master.

Yeah, Greybeards will literally blow you off the mountain.

And Dragons are easier to kill than Giants. Because Giants will knock you into the atmosphere.

Then you haven't lived.

http://i1209.photobucket.com/albums/cc398/buckomccane/Skyrim/BDD_01.jpg

Dragons are easier to kill than "my character" because my assassin is a hard-ass. lol

I couldn't think of anything more creative.

yeah I've played over 350 hours unfortunately everytime I was about to fight two dragons there was a glitch or it would not land

I saw a Free-for-all battle of all the creatures in-game.. Well, 5 of them actually.

Ancient Dragon won 2/5 of them.
Dragon Priest won 3/5 of them.

Briarhearts. Because those guys are just crazy.

Briarhearts are crazy but they are very easy to kill. Actually, the Briarheart in those FFAs died very quickly. but he did a lot of damage to some things and manage to kill a few things.

(Off-topic) Nice avatar Arcona Cor.

I was quite shocked when I one-killed a Briarheart with my level 30 spellsword. I couldn't get close to them on my first character, and that was up to level 50 or so.

Edit: Of course, his Ravager pal totally kicked me afterwards...

(Off-topic) Why, thank you!

Yeh, I know. I kill them in about 4-5 hits very easily. Only thing you need to worry about them is when they do that duel-wield move. They can do like 100-150 damage with that move on Master... And I'm wearing Full Daedric.

Yup. Their attacks can be critical. Especially awful thing is when they switch to ranged spells after running away.
The best tactic I've found for killing them is sneak attack + strongest spell I can find + more hits if they're not dead yet.
Also, I usually take them down first, since they tend to surprise-kill me while I'm fighting the others.

Whenever I enter a Forsworn place, I take them down first as well while Cicero and Derkeethus work on the weaker ones. Or I summon two Dremora Lords right on him and watch him get slaughtered by those guys.

Dragons are pretty hard for me I met a frost dragon just yesterday and had to run, there was no beating it. Blood dragons are easy though.

Just pickpocket the Briarheart out of the Briarheart's inventory. They die instantly. Turns out that Briarhearts are undead, and are also affected by the Restoration Perk that increases effects of spells against undead.

But if you can get close enough to a Briarheart to get a sneak attack, just pick pocket the briarheart (briarheart the ingredient) and problem is solved.

Their sneak detection is absolutely ridiculous, though.

Not at 80+ Skill level, requisite Perks, and the Shrouded Armor it's not...

Well of course not, but relative to other NPCs they have ESP or something.

Yeah... all Briarhearts are secretly Psychics enlisted as Elite strike teams by the Psijiic Order. You just happen to stumble upon them at really bad times...

lmao

OMFG, that's f-ing hilarious.