As far as the original manga goes (no super... DB only... Which is the only thing I'm interested in lulz) Kuririn was likely the strongest Earthling. Yamcha stating to himself that "among earthlings, at least [Kuririn being the most powerful]) I think is the most concrete evidence to be the author using a character as a tool to relay the info of who's the strongest Earthling. Tenshinhan had some cool entries and all, but mostly with special attacks under special off guard circumstances... It seems that Elder Namekian bringing out Kuririn's potential changed the status quo and made him a power Tenshinhan couldn't close the gap.