Base game, dragon priests can be tough just because they're hard to get close to if you're a melee type and they can deal high elemental damage while you chase them around. Also, being swarmed by Falmer.
In Dawnaguard, the snow elf at the end of the MQ was pretty tough. The Charrus Reapers are a pain too, that poison is a *****.
Base game: Morokai. Dragonborn: Miraak is the toughest I've faced because I can't run away, heal, return like I could other boss fights. Dawnguard: Harkon, though Vampire Lord versus Vampire Lord was fun.
Next up on the Packers' hit parade: Packers Off Season :( http://socialmediabar.com/andersons