Superman. Unless Alduin had Kryptonite Breath.
I don't know. Seems like it would be a standoff to me. Superman, assuming he is the comic book Superman and not also dragonborn, cannot permanently kill Alduin. I'll give Superman the benefit of saying that he, unlike the Dragonborn, does not need to use dragonrend to harm him. After all the guy can tether planets to himself and pull them around, so I am fine with Superman being able to falcon punch Alduin back to Sovngarde without the Thu'um. However, once in Sovngarde, Superman cannot permanently kill Alduin without dragon blood.
Alduin, on his part, will be completely ineffective against Superman, as the Man of Steel is vastly stronger, faster and more durable. So, short of getting his hands on Kryptonite or somehow being able to turn the yellow sun into a red one Alduin would be like big spiky gnat to Kal-El.
