4. Jo Grant
Jo was one of the Doctor's most adorable companions. Succeeding Liz Shaw, she was the exact opposite of a fierce scientist almost equal to the Doctor. Sweet, and kind, and just a little prone to being clumsy and getting captured. She stayed with the Third Doctor for most of his run, and is arguably his best known companion. It's fitting, then, that Jo's departure is both sweet and heartbreaking. Occurring in The Green Death, she falls in love with a scientist and decides to marry him. A step which involves leaving the Doctor, for once and for all. The realization going across both her and the Doctor's face when they both understand this is deeply sad to see, it's a goodbye that seems to have personal meaning for everybody. And this is quite obvious every time you watch it. You can obviously see that the Third Doctor is absolutely heartbroken to say goodbye to Jo. Pertwee doesn't go into huge hysterics, but his understated performance is all the more effective because of it. You get a genuine sense of his heartbreak at having to say farewell, even if it's to send her off to bigger and better things. Such overt affection only makes the manner of her departure even more touching. Because she really is going off to bigger and better things. Jo's goodbye is heartbreaking, but it's actually one of the happier ones. She's leaving for love, to fulfil her own dreams. She's saying goodbye to the Doctor, yes, but there's a large implication that she's going to be happy even without him. She's still going off on adventures, she's still going to be essentially Jo. Her ending is in character, and wonderfully touching. It isn't devastating on a grand scale, it just suits Jo and her personal journey perfectly. Jo Grant is probably the most famous companion of the Third Doctor, and so she deserved a wonderful send-off. She got this in a departure that was both heartbreaking and hopeful, one that obviously affected every single member of the 'TARDIS team'. The Third Doctor's era wouldn't have been the same without her, and her ending shows this absolutely perfectly.