3 (tie). Mork & Mindy (ABC)
Robin Williams made his debut as the alien Mork from Ork on an episode of Happy Days in February 1978, and by the following September, he was headlining this spin-off. Though this first season earned both him and the series Emmy nods, critics and fans promptly fell out of love when the cast and direction were tweaked for Season 2. Matching its more serious tone, Mork & Mindy took a serious drop to No. 27 in the ratings, then remained outside the Top 30 for its final two years.
Seasons on DVD: First three