Assassins. Because mages don't have spells that can be "smithed". The potions only help mages so much. An assassin with a powerful bow can kill things before being touched while wearing decent armor. And they don't have to die if caught. Characters can achieve 100% spell absorption and 80% physical reduction with spells/armor.
Moral of the story: the smartest character is the most OP character.
Necromage + Fortify Enchanting Potion = Powerful Fortify Smithing Set
Necromage + Powerful Fortify Smithing Set + Fortify Smithing Potion + OneHanded Perks = A dagger with ~300 dmg per hit.
Shrouded Gloves + Assassin's Blade = ~9000 dmg per hit. ~18000 if used Critical Charge while sneaking.
You can do ~33000 dmg if you use all the words of Marked For Death on a foe and then use Critical Charge after 60 seconds. It can be effective even if not using sneak if the player has enchanting perks that boost fire, shock & frost enchanting and also destruction augment perks. All you have to do is enchant a stalhrim dagger with chaos damage and absorb health/paralyse.
If not using Sneak, I'd say any mage with Alchemy, Enchanting and Destruction.
My board 'n sword paladinish mage was pretty OP, doing ~450 dmg per hit and blocking even the strongest blows. Also having maximum magic resistance and armor rating. If you have the Shield Charge perk, then you're unstoppable. I was using destruction when I wanted to deal from afar. My armor had %100 Fortify Destruction so spells cost no magicka. And if you have a set of Fortify Alchemy set and make Fortify Destruction & F. One Handed potions, Lightning Storm is quite devastating against dragons.