Board Thread:Roleplaying/@comment-24339224-20150816215930/@comment-1470368-20160203161535

None of them, to be honest.

Atmoran, Nord, Imperial, and Breton are all just as likely to have been Tiber Septim's race (only if you believe certain sources, and the Lore is quite contradictory)

To put it simply, Race doesn't matter in the context of the Dragonborn. You're a mortal gifted by Akatosh with the same power of the Dragons.