What do you think is the most powerful mage follower? I mean like OP as [censored].

Edit: Oops, misread the title, I guess.

Marcurio. He destroys EVERYTHING. I send him home because he wouldn't let me kill anything. Expert difficulty if it matters.

There was a discussion about this in gamefaqs http://www.gamefaqs.com/boards/615805-the-elder-scrolls-v-skyrim/61139818

Last one I'll note, is everyone's favorite Khajiit mage, J'zargo. He is without a shadow of a doubt the best Destruction mage in the game. His level caps at an unreachable 34,464, which essentially means he'll continue to level with you post 50. However I took his stats at level 50:

Health - 458
Magicka - 325
Stamina - 132
Alteration - 15
Conjuration - 15
Destruction - 100
Enchanting - 15
Illusion - 100
Restoration - 60
Archery - 20
Block - 15
Heavy Armor - 96
One-handed - 100
Smithing - 15
Two Handed - 15
Alchemy - 20
Light Armor - 15
Lockpicking - 20
Pickpocket - 20
Sneak - 25
Speech - 15

for me it's Aranea Lenith or Illia..both we're kinda hard to get but it was worth it..