First and foremost is total population.  Whether high skilled or low skilled, the more bodies we have, the more resources they consume.  So slowing the overall population growth rate should be the top priority within the immigration quotas.  Then WITHIN those quotas, I'd deal with the "quality" issue you rose (i.e. giving preference to value-added immigrant applicants over less value-added).

Of course, that's all LEGAL immigration.  The obvious priority above all that is stopping illegal immigration dead in it's tracks.