I'm going to go with the Waters of Mars to me this episode is so engaging with so many great ideas in a way the episode is very similar to the fires of Pompeii with the doctor in a very important point in time having to decide if he should save anybody and possibly spoil the timeline or just watch it happen and the episode also introduces us to the time lord victorious ark where did the doctor it becomes a antihero like character train his best to do what he thinks is right and the doctor's line of the rules of time are MINE AND THEY WILL OBEY ME!! Speaks so much for the doctors character and even makes him become very similar to rasalon