Hmm, well the way it could go either way honestly. Aang would be on the defensive, he almost never fires the first the blow and it really isn't his first nature to be offensive. Superman would probably beat him around, alot. (This fight dosen't really fit into either characters personality at all so its kinda weird). When Aang is near death the Avatar state would activate and most likely catch superman off guard and land a couple good shots on him. From here it all depends if Aang decides to blood bend or not. If he does, he might win. If he doesn't then Superman defiantly wins and the Avatar Spirit would die because he was in the Avatar State.