From my take on the lore, here is what I've gathered. I'm not saying it's completely accurate because I'm still learning.
High Elves or Altmer are and remain the #1 magic users for now. Saying that they're naturally gifted in the arcane arts is an understatement.
Dark Elves or Dunmer would be the number two strongest magic users. If you're dumb enough to insult one, chances are they will melt your face off.
Bretons, being half Mer and half Man, come in third. While Bretons can rival High Elves in intelligence, their magic are considered cheap tricks in comparison.
Any other race has the potential to become a master mage, including Orcs. In the College of Winterhold, the only race I don't see there is the Argonian, but I assure you they can match an Orc in magical combat.
According to lore, If a fight were to break out between a master High Elf Wizard and a master Orc Mage, I would say the fight would be an uphill battle for the Orc, but I'm not saying it's a 100% death sentence for them.