Captain America. He is the mortal that gods defer to.
Have you ever read the Earth X series of stories? I believe it was the second one that had the war in the afterlife. When all the heroes stood on top of a ridge over-looking Death's throne and Thanos, they all stood there until Captain America lifted his shield into the air, one-armed, without a single sound. Then they charged. It was an extremely inspiring panel and has become one of my most favorite moments of comic book history.
And that right there is why Cap, to me, is the mightiest.