Arena (Eternal Champion): Must've been pretty badass. In a journey that apparently spanned roughly ten years (maybe seven or eight, but Ria's dialog in the game implies she contacts you shortly after Tharn usurped the throne), this lone character travelled the whole of Tamriel delving into some of the darkest and most dangerous places, with little more than the advice of a dead woman to guide them. I'd say, if they still live, they must be one of the most powerful mortals to walk Tamriel at this moment.
Daggerfall (Septim's Agent): A very resourceful and cunning person, considering they were able to wade through the political turmoil of the Iliac Bay and not get assassinated in the process. Whilst the dungeons in Daggerfall were really something, the Main Quest ones are probably the only ones worth considering canonical, and they weren't particularly challenging since they actually had logic in their design. Septim's Agent may or may not be a physically powerful character (depends on how you play the game), but I'd say they had quite a bit of wit and intelligence, being able to survive in such a large world where everybody is a nobody and factional alliances are everywhere.
Morrowind (Nerevarine): Whereas the Eternal Champion seems to be physically powerful and Septim's Agent intellectually so, the Nerevarine is probably the most well-rounded. Starting with nothing, they seized the day and gradually climbed their way through the Vvardenfell hierarchy of trust and became an incredibly powerful individual. The catch? Lots of divine intervention on the Nerevarine's side - Azura, Talos, and possibly even Vivec's CHIMDUDE influence. Now that the prophecy is fulfilled, sure, the Nerevarine may still retain much of their power from their adventures on Vvardenfell, but just how capable are they now, alone?
Oblivion (Champion of Cyrodiil): This is a tricky one as you served more the role of an attendant and sidekick in this game, which is a refreshing change of pace from constantly being part of some "epic" plot line, but at the same time, there really wasn't any emotional burden nor real development of character in this game. It was more of a linear adventure of action, accomplishment, and great deeds, less about politics and self improvement. To end on a high note, singlehandedly demolishing the planes of Oblivion has to count for something.
So, who do you think is the greatest? I have to vote for the Eternal Champion, because whilst all the protagonists in The Elder Scrolls are pretty powerful (s)he is probably the one who accomplished the most without divine intervention. A very persistent and strong-willed adventurer who had nothing to rely on but Ria's advice and their own motivation.