It depends, if we assume they're all just warriors.
- Nerevarine = I would say that this guy is special in any sort of way, I mean, he's the champion of Azura so I guess that gives him a few tricks but I think a normal person could be as good as them in combat with enough training. However, he did manage to beat an ASPECT of Hircine will I assume is no small feat. They also took down Almalexia, one of the three Dunmer gods, and arguably the most powerful of the three.
- Champion of Cyrodiil = Well, I always assumed that what you took over Sheogorath's role, you don't actually become a God but have the title of Mad God and have an extended influence over the Shivering Isle - like Sheo's second in command while he is away on vacation. In the main story, you didn't do much expect run errands for the TRUE hero in my opinion.
- Dragonborn = Now I think this person is probably the most powerful, due to their knowledge of Thu'um, which would give them a MASSIVE boost in combat efficiency. Plus we've not yet delved into the expansions yet so we might see some interesting things happen to our little dovah.
So, my judgement comes down to either Nerevarine or Dovahkiin if we're going of melee combat on a fair plane. HOWEVER, if the fight took place in the SI, The Champion of Cyrodiil would win hands down.