Almost all permanent research staff at universities are professors. Maybe the confusion is because some countries don't use that title? Regardless, I'd say the

Almost all permanent research staff at universities are professors. Maybe the confusion is because some countries don't use that title? Regardless, I'd say the definition of a "successful researcher" is usually someone who has become a professor. Whilst there are many great postdocs, very few (any?) of the good ones become professors.