Now into the top three moments and this moment is one that's particularly difficult to come to terms with, even after all this years. If you hadn't already guessed from the heartbreaking picture above, it's the moment Donna Noble was forced to forget. With the human/Time Lord metacrisis burning up her being, the Tenth Doctor had no other choice but to completely remove the memories of him and their travels together from her mind. The Doctor knew that this would happen and maybe before undergoing this irreversible deed, he tried to think of an alternative way to keep this version of Donna alive. Alas, there was no other way. She was going to be with him forever, and maybe in some parallel world, she will be.