Definitely Johnny Carson. He was on TV for what, four decades? From the days of black and white right through the dawn of CGI -- he first went on the air in 1953 and retired in '92 while Babylon 5 was preparing to usher in a revolution in computer graphics on television.
At its peak, more people watched the Tonight Show with Johnny Carson (which ran for thirty years) than any other show on TV. He and that show were so popular that for decades, sitting in bed watching Johnny before going to sleep was a national pastime in the US and Canada.
Johnny was the king of TV, nobody before or since has even come close.