Most likely the Earthlings for reasons sei'taer mentioned. There would probably be a lot more Earthlings with similar battle power if given an equal opportunity.
That's only if we're thinking of "potential" in terms of peak power, however, since Namekians have far greater potential when it comes to unique abilities as well as methods of achieving greater strength such as Namekian fusion, whereas the Earthlings tend to be limited to either training hard or honing a deadly technique.