It all depends on your character and how you've built it. Miraak is definitely up there if your higher level, at lower levels the bandit chiefs can be tough especially when they 1 hit kill you with a stupid animation.
For followers the female mage that you can get from the azura quest is the best mage follower in the game i believe as she uses the highest level spells, as for warriors i would say its that woman from the skaal village in dragonborn that dual wields for stealth probably the dark brotherhood initiates.