The only thing in the game in which you can sneak behind to kill
Forsworn Briarheart
100
It is a popular meme with a dragon and the Dragonborn in a face to face stand off.It is also a very common material if you are a serious adventurer it also requires smithing 100.
Dragon Bone (no matter what weapon)
100
You get this from escaping a prison and teaming with the enemy. And has a familiar look to it and has a lot of ties to Markarth.
Armour of the Old Gods
100
Natives of Elsweyr they have the unique ability of claw damage. They are the ideal sneak race.
Kahjiit
100
He is the most pimp vampire on the planet Nirn (excluding Serana because she is P.I.M.P) and in the battle that you must fight him he will be a vampire lord during the battle.
Lord Harkon
200
Only difficult for low levels and if you don't have a fire based weapon. You can find one of these as soon as you travel to High Horaothgar.
Frost Troll
200
You get this during the dawnguard quest line. It has the ablity to make light go bright or go dark.
Auriel's Bow
200
It is very common like in the last category if you are truely the Dragnborn.
Dragon Bone armour
200
The are the natives of Valenwood and are quite good with a bow. They are also very ideal for sneaky characters.
Wood Elf
200
Why you know who it is the Eater of Worlds, Destroyer of mankind, and The Slayer. This ferocius opponent will make your head spin with the power of his shouting. ( The Black Wings Unfurled).
Alduin
300
You must be at least level 70 for these to spawn and are very hard no matter the difficulty.
Legendary Dragon
300
This is a godly weapon that any one with high stamina will enjoy and if they want to live forever in Skyrim. It sounds like a shield but is a simitar.
Windshear
300
This is the holy nocturnal armour (hint hint) it has its own unique stats and then has benefits of looking insanely cool.
Nightingale armour
300
These fellows are native of Black Marsh and have the amazing ability of regenerating 10x health for 60 seconds. They are fit for the thieves.Argonian
Argonian
300
Said to be the first Dragonborn he will be the hardest persn so far on this list with him constantly refilling his health by killing dragons close to him makes you defeat 4 times in total.
Miraak
400
These start spawning when you reach level 20 and are very fearsome. They can be named and they can be found as last bosses in dungeons.
Dragur Deathlord
400
This weapon has the unique enchantment of being able to have a higher chance of 15x damage than any other weapon
.Valdir's Lucky Dagger
400
You must wait and spend a lot of in game money to get this armour and is the only set with waterwalking.
Azhidal's Armour
400
They are the brutes of Tamriel and take no crap from any one who stand in their way. They can also have the ability of taking half damage and dealing double for 60 seconds.
Orc
400
He is the Redguard hero and he will challenge you once you reach level 80 in a hold capital ( Whiterun is best for him to spawn) and you must travel to his Last Vigil to send him to Sovangarde.
Ebony Warrior
500
They can literally slow you down and are very strong with their magical abilities. They have a fudge ton of magika.
Arch Mages (depends on what their specialty of magic is)
500
This item has the ability to absorb all attributes at one time and you have to have the Dragonborn DLC pack.
Dwarven Black Bow of Fate
500
To get this set you must have the Dragonborn DLC pack and it will take you on a long journey to find ancient treasure chests you also get two weapons on this quest.
Deathbrand armour
500
They are the face of Skyrim (literally) and are the natives of Skyrim as well. They have the unique ability to make enemies flee for 30 seconds.
Nord
500
The most pimp of all of the of the quest bosses and right when you start up the game he is already level 80 and he
has double the Ebony Warrior's stats. You need the Dragonborn DLC pack to access this pimp boss and you shouldn't think about taking him on until at least level 90.