Before there were Grey’s Anatomy, House and Scrubs, there was ER. The show that made George Clooney a star (thank you!), goes down in history as the longest-running medical drama ever, with a record 15 seasons. After its September 1994 debut, the show achieved massive popularity, ranking at the top of the Nielsens for many of its early years and garnering more than 40 million viewers at its peak. Key word there: peak. The cast members came and went almost as often as patients do in an actual emergency room (Noah Wyle was the only star who stuck around for the entire run), and viewers found it hard to hang on after each departure. NBC should have taken a lesson from those stars and said goodbye before the show flatlined.