Realistically, it would depend on how advanced the dragonborn is I think. The Dragonborn would have to be like level 100 to stand a chance against the Greybeards. The reason why only one of the Greybeards talk a good amount is because, as said Greybeard explains, a single whisper from the others could kill. That's why if you try to talk to one of the others and they whisper "Dovahkiin." the temple will shake and slightly crumble. It's a tough one, but I think Greybeards win simply because of their shouting capabilities. The Dragonborn is more active with combat though, so again, it's a tough one. EDIT: Didnt even consider the options for a second. Oops. Oryx definitely wins. He's a god that can only be killed by 6 guardians almost perfectly getting through all of his servants and doing his own puzzling pattern to kill him, and even then he respawns back in the Dreadnaught after death.