7. Regis Philbin
By Dave's count, Regis has been his most frequent guest, appearing 136 times. Viewers knew what to expect when Philbin would come on: Dave would harass him, Regis would get defensive and yell right back, and then maybe he'd would tell Joey Bishop stories. It was old-school talk-show shtick, but the affection beneath the animosity was apparent and endearing. And it was a sign of how much Letterman relied on Philbin that he had him on for that first somber Late Show after 9/11: "Thank God, Regis is here, so we have something to make fun of," he told the crowd.