Nerevarine hands down, can levitate, be 100% chameleon (think invisibility but not broken by attacking or doing anything like the crappy invisibility we have now), jump across Morrowind in 1-2 jumps, run across Morrowind in a few seconds, can enchant so powerfully you can command gods to follow you around, killed around 4 gods in his day, can drink an infinite amount of potions at once, including strength enhancers, has poison spells, spears, 50meter AoE spells, can combine more than 4-5 spells into a single attack, I could go on but no need.
the games don't do a scan on your old saves they just assume that the previous character was the default for that game. I.E. Nerravarine is a dark elf, CoC/Sheogorath is an Imperial, I'm sure in TES 6 Dragonborn will be a Nord.
The games don't assume anything.
On-topic: Torn between Dovahkiin and Nerevarine. I think I'll give it to the Dovahkiin.
Until you spread your wings, you'll never know how far you can walk. GT-- The Baconer
My rationalization for Sheogorath and the Champion being one and the same is that the Princes seem to able to choose their form. The Champion probably reverts to the "Gentlemen with a cane" appearance of the former Madgod when walking the mortal world, partly for convenience so as to be easily recognized by followers, but also to at least TRY to keep their own legacy untainted. I imagine it would be a hell of a blow to find out one of your greatest heroes ever became the god of insanity. <_<
That is also why I'd say the Champion would win against Nerevarine and Dragonborn. The two might be tough, but NEITHER has become a full-on god. All the Champion would have to do is whisk them away to Shivering Isles where he can fully bend reality to his whim, and it's over.