For completing the College of Winterhold quest, you'll be rewarded the Archmage's Robes, a powerful set of robes that grant the player a 15% reduction to spell cost for all spells, +50 Magicka, and +100 Magicka regeneration. If that wasn't enough, you get the Arch-Mage room in the College of Winterhold, equipped with Arcane Enchanter, Alchemy Lab, and all kinds of potion ingredients and Soul Gems.