3. Adric (1980-1982)
Introduced near the end of the fourth Doctor's reign, Adric immediately became the most hated companion up to that point. A boy genius, he was written as being arrogant, abrasive, and immature. Despite the fact Adric could've been a great character (Matthew Waterhouse, who played him, is a pretty decent actor), the writers apparently saw fit to ruin him. When it finally came time for him to leave the series, this time the writers saw to it he had a proper and memorable exit. In 1982's Earthshock, he's trapped on board a ship overrun by Cybermen, which has also been set to crash into prehistoric Earth with the controls locked with a complex puzzle. Barricaded in the control room, Adric tries to solve the puzzle to unlock the controls, and is on the verge on completing it when the Cybermen finally break in and shoot the controls, destroying the puzzle. The camera does a closeup to Adric's face as he watches Earth getting bigger and bigger on the screens. Finally after two years, the writers managed to make use care about Adric. Just before they killed him. Drat.