4. Varsh (Full Circle)
Whenever I watch Full Circle, I'm always filled with regret that Adric's brother didn't board the TARDIS in his stead. Don't get me wrong, I hate Adric a lot less than most people do. But he just wasn't the right fit for the group. They already had three genius-type characters in the Doctor, Romana, and K9, and would soon be getting another with Nyssa. The TARDIS could have really benefited from the contrast that Varsh's rough and tumble, shoot-first-ask-questions-later approach would have provided. I loved his natural leadership, and I feel that, like Leela, he would have grown so much under the Doctor's guidance. In this case, it just feels like they totally picked the wrong brother. Varsh was an interesting, multi-faceted character, whereas Adric (God love him) was just another teen genius stereotype.