American Idol
For an unprecedented eight consecutive years, from the 2003–04 television season through the 2010–11 season, either its performance or result show had been ranked number one in U.S. television ratings. The popularity of American Idol however declined, and on May 11, 2015, Fox announced that the series would conclude its run in its fifteenth season.

Q: How many consecutive years did American Idol been ranked number one in ratings?
eight