5. ER
Debuting in 1994, the NBC medical drama, which made George Clooney a star, followed the doctors who worked in the emergency room at the fictional County General Hospital in Chicago. The show ran for 15 seasons and toward the beginning was able to achieve extreme popularity.
According to the Los Angeles Times: “For those first five seasons with George Clooney onboard, the show averaged an audience of just less than 30 million. Even for the next five seasons, the Thursday night staple still averaged a viewership of almost 22 million. Beginning with Season 11, the medical drama dropped out of the top 10.”
By Season 14, the show’s viewership had fallen below 10 million for the first time. Through the years, the show lost much of the original realism it had become well-known for. Cast members came and left too quickly, and viewers began dropping the show as a result.