Those two are obvious contenders. you must also take into account the time, heavyweights were much smaller then so while I'm sure todays fighters would wallop them, they still don't come close.
I do think though that both Joe Louis and Rocky were fairly static fighters, especially Rocky.
I think from the clips I've seen that Johnson was a better mover (and decades earlier too) although didn't have their knock out power.
I know Foreman says Louis best ever heavyweight.