Altmer would have to be the most magically talented since they are the closest thing you can find resembling Aldmer, more than any other race. And the Aldmer are direct descendants of the Et'Ada. While all mer races descended from the Aldmer, the High Elves are the most pure and unchanged form, whereas the other races have evolved into something entirely different.
And the Bretons do trace their ancestry back to the Aldmer, but their blood is heavily mixed with the blood of men. As a result, they are extremely resilient to magic, but not as skilled.
Yes, but what about their actual power? the starting stats at the beginning of the game (like +10 to conjuration for Bretons, +10 to illusion for the Altmer) doesn't really matter as you go on ahead with the game and level up your stats. However, the +50 Magica is very useful when playing as a mage as a Altmer. On the other hand, the Bretons have a 25% magic resistance, which can be combined with skills from the Alteration tree that upgrade your resistance further, with the Atronach Stone, the Agent of Mara blessing, and your racial power, where you have a 50% chance of absorbing all the damage from a spell. You can actually make it possible to be invulnerable to all magic for a short period of time. They can also use an extra 5 leveling up points to come to the level where the Altmer are at. Both are useful in different ways, but I prefer to use the Breton as a mage. Another thing to factor in is what you are going to specialize in as a mage. I focused on Conjuration and a bit of Alteration, starting with the bound sword and the flame atronach, and then using the storm atronach when I got it. if I wanted to be more of a Destruction specialist, I would probably want to go with the Altmer. More magica will be needed if playing as a Destruction mage since you would be constantly using spells like flames and sparks to deal damage. if you are playing as a Conjuration Mage like me, all I needed is enough magica to cast flame atronach and bound sword. i also needed to put in some one-handed perks to deal more damage with the bound sword, but that's another story.