8. The Impossible Planet/The Satan Pit
One of the scariest and most suspenseful episodes of the new, revived series was this two-parter near the end of Series 2. Landing on a planet somehow gravitating around a black hole, the episodes introduce the Ood, who go on to reappear in later series, and the Beast, who labels and torments the Doctor. The isolation of the planet where the episode is set, and the threat which faces each and every character, genuinely scared me when first watching this as a 10-year old - the conclusion was also unexpected, as was the meeting with the Beast. It might be a far-fetched story, but its incredibly well-written and one of the most memorable episodes of the era. The Doctor as well its at his best, going from excited, childish science-enthusiast when dealing with the base, to a scared, angry, raging and scarred man, particularly in the stand-off with the aforementioned Beast. The scene is memorable and emotional, showing just how torn and emotional this Doctor can get. And intercut with the panic which is taking over the base above, as well as the Ood who now are determined to kill the human explorers, makes it a thrilling and fast-paced adventure for the Doctor. As a two-part story of new Doctor Who goes, it is probably one of my favorites, and certainly one which will live long in the memory.