Dovahkiin. I haven't played Morrowind in a long time so I'm really fuzzy on lore but we need to view accomplishments and powers:
Dovahkiin
- Defeated the Self-Proclaimed Son of Akatosh
- Has Dragon Shouts and can master the Shouts very easily
- Has a Pet Dragon
- Ability to summon the Three Original Heroes from the Dragon Wars
- Went to the Realm of the Dead, survived, and faced down Tsun
- Looked into the Elder Scrolls without going permanently blind
Champion of Cyrodiil
- Helped an illegitimate child of the Emperor defeat a Daedric Prince
- Defeated a Daedric Prince
- Became Sheogorath
Dovahkiin would win. Although the Champion of Cyrodiil defeated a Daedric Prince, Dovahkiin defeated the World-Eater and a semi-immortal Aedra like Dragon.
Dovahkiin was unable to kill Alduin alone, he had to enter Sovngard and get help from the three heroes in order to defeat him. not to mention i remember reading somewhere that Alduin was only delayed, and that he will eventually return (not sure if that is true or not)
Dovahkiin may be a hero of Akatosh as people say, but the CoC is the champion of The Nine (Knights Of The Nine), meaning he is also a champion of Akatosh as well as the other 8.
where the dovahkiin required help to defeat Alduin, the CoC managed to single handedly defeat the Daedric Prince of Order (Jyggalag), there by becomeing a daedric prince himself.