I have to go with Bretons because they are definitely the best race Skyrim has. Their resistance to magic and ability to absorb Magicka says it all.
I think the Argonian is the best race. The Argonians, are built for thievery. They feature favored skills towards Lockpicking, Pickpocket, Sneak, Alteration, Restoration and Light Armor. They start off with the Flames and Healing spells, and regenerate health 10X faster for 60 seconds thanks to their Racial Power, Histskin.