The one where Weng-Chiang's true identity is revealed...
Tom Baker is so 100% The Doctor in this episode, commanding the screen with the greatest of ease and dominating every shot he's in, never mind every scene. It wouldn't be long until Baker's interpretation of his Doctor would slacken a little, soften up and become less commanding, but this really feels like one of the final examples of this version of the Fourth Doctor. The first meeting between Weng-Chiang and the Doctor is delightful, with Tom switching from light and playful to deadly serious effortlessly. "What have you done to her?" he asks of Leela. "Nothing, yet," teases Weng-Chiang, to which a steely Doctor replies: "Take my advice. Don't."
You really would not cross this Doctor, not this version for sure! It's another example of how the Doctor seems to have developed a great fondness for the savage Leela. He expressed concern for her several times in The Face of Evil (even throwing a flesh-eating monster at someone for slapping her across the face), and here his concern shows through again with a barely veiled threat.