I don't see the appeal with King Hippo really. He's not very mobile to be an AT. I can see him as a boss more so.
These are my choices.
Bald Bull
To me he seems more of Little Mac's rival than any other character. He has appeared in all Punch Out games (with Hippo only in two). He was also in Warioware game and arm wrestling game. Just like his barber he doesn't know when to quit. He is big and really fast with his bull charge gives an instant K.O.
Mr. Sandman
The only other person I could think that could be consider Little Mac's rival would be Mr. Sandman. He's also been in every Punch Out game and is usually one of the toughest contenders in the series. Usually when you see parody movies Sandman is the man to beat.
Hoy Quarlow
This guy has the craziest fighting style in the Punch Out series. Using his cane he would bounce off the ropes and hit you from all angles. If we're going to have a SNES rep I would want him.
Mike Tyson
I didn't choose him because he's a real person. A nightmarish character with lighting fast instant kills uppercuts. Though I think he would be a very good boss, personally.