Dovahkiin no question.
Because I'm fairly versed in the beyond-the-game lore of Tamriel, I'll spell it out.
The Nerevarine is an incarnation of General Nerevar Indoril. Now while Nerevar was quite the warrior, given that he took down slews of angry Nords in his day and at the Battle of Red Mountain, he was an accomplished diplomat as well, as he was Hortator of the five Great Houses and united the Ashlander clans against the Nords and Lorkhan at Red Mountain. That being said, he could only be summed up as an extraordinary soldier and commander. He also had Azura on his side, and that doesn't hurt either.
Dovahkiin on the other hand is the crux of greater power. He is what you would call Shezzarine, or an incarnation of Shezzar, or Shor as the Nords call him. Shor is nothing short of the greatest warrior to ever set foot on Nirn, as he is the Heartless, living embodiment of the Missing God Lorkhan. In fact they are the exact same. After creation or Convention, Lorkhan was killed and his Heart ripped from his body. The Heart, as any Morrowind fan will know, was deposited in Resdayn, later called Morrowind by the Velothi pilgrims who would eventually become the Dunmer. One of the Towers, Red Tower would grow around the Heart, and Time would begin. Lorkhan's body would stay in the sky though, becoming Masser and Secunda, the twin moons of Nirn. However after Lorkhan's death, he would appear on Nirn as a Heartless warrior-wanderer. The Nords would call him Shor, with the Cyrodiils calling him Shezzar.
Later on however in the first era, the first Shezzarine would appear in the form of Pelinal Whitestrake, as seen in Knights of the Nine. Pelinal would go on to help Alessia and Morihaus defeat the Ayleids or Heartland Elves and create the first Empire of Cyrodiil.
Another Shezzarine would come as Talos Stormcrown, the High Rock general who would help King Cuhlecain at Old Hroldan and eventually conquer Colovia and later use Walk-Brass Tower or Numidium to siege Alinor and conquer the first Aldmeri Dominion, thus conquering all of Tamriel, save Hammerfell and Morrowind (as Morrowind was allowed autonomy because the Tribunal gave Talos/Tiber Septim the pieces of Numidium).
Dovahkiin is Shezzarine as well. His power is that of Talos which is to say the power of Shor, and Shor is among the most powerful beings in TES universe.
If Dovahkiin was allowed even a sliver of the Thu'um that other Shezzarines like King Wulfharth and Talos had, Skyrim would be so easy it wouldn't even be funny. Talos' Thu'um could repel arrows, and Wulfharth could swallow storm clouds and Shout people blind and out of existence. In the Five Songs of King Wulfharth, he is even described as Shouting Time apart to age his people out of infancy when Alduin ate their years.
Even with Morrowind's uber-player mechanics, a master of Thu'um is unbeatable. When Talos left High Hrothgar, the Nords called him Ysmir because they no longer saw a man but a dragon.
Welcome to the world of crazy, over-the-top TES lore.
