The one where the Daleks turn on their creator, Davros...
I didn't notice at the end of part 5, but in the reprise I heard the Doctor scream Sarah's name in agony, which just makes the whole thing even more upsetting. As he's being throttled to death by not one, but three Dalek mutants, he calls out for his best friend, Sarah Jane Smith. Not macho man Harry, but Sarah. How touching...
Genesis of the Daleks has a few moments which have passed into Doctor Who legend, but chief among them - and one of the most legendary of all Doctor Who scenes - is the "Do I have the right?" scene which follows here. The Doctor questions whether he has the right to commit genocide, to wipe out the entire Dalek race, simply because they will go on to destroy so many other other peoples and races. Surely, that would make the Doctor just as bad as they are? Sarah is the audience's voice of reason here, reminding the Doctor what the Daleks are capable of, but then he puts to her that old time traveller's favourite, would you murder the child Adolf Hitler so that he didn't grow up to slaughter millions as head of the Third Reich?