I absolutely refuse to consider the Champion of Cyrodiil to be the same person as the one who went into the Shivering Isles, both because I feel like that's too much for one person to do and because that's not what I consider canonical for my player-lore. There's also nothing I know of that implies the CoC became Sheogorath.

There's nothing that leads me to believe the Nerevarine is all that exceptional in power either. Sure, he/she did a lot of cool things and fulfilled some meaningless prophecies, and there was the awesome feat of entering Red Mountain, disenchanting the Heart of Lorkhan and escaping/defeating Dagoth Ur, and I would never dream of undermining that accomplishment, but that does not require any exceptional amount of power.

The Dragonborn, however, is nearly a god amongst men. He/she can use the Thu'um without any dedicated training. He/she can absorb the soul of dragons, killing them for good. He/she is an exceptional being, whom, by blood alone, is more powerful than the average mortal, and it does require power to defeat Alduin, The World-Eater in combat twice.

It is hands down the Dragonborn, but that's taking it from the standpoint that the Nerevarine, CoC, and Dragonborn ONLY accomplished what is required to complete the main quest, because, according to lore, that's all they did. If we were to say that the CoC DID become Sheogorath, then Sheogorath is obviously going to win, but then it's no competition.