This would be an absolutely great fight until Liston gets Rocky'd measure and batters him in 9 or 10 rounds or Marciano grits his teeth, gets on Liston's chest and bullies him down the stretch.
I can't really pick. The reach disparity would be too much and I think The Rock would get too busted up trying to close the distance. He'd get his like in, but would get mangled and stopped due to cuts/damage.
Cop out answer but I just can't figure out what it would look like if it went to the championship rounds