3. Renaissance Florence: 1480-1503
This is not just an Assassin's Creed obsession I've got here. No, Florence in the Renaissance is genuinely very good ground for a Doctor Who episode. The look of the places and costumes lends itself to a period drama style episode. Character-wise, you've got minds like Leonardo Da Vinci and Niccolo Machiavelli providing company or foils for the Doctor, and the introduction of big ideas (a useful caveat for any science fiction). I especially like the idea of the Doctor having to team up with Machiavelli, leading to a tense and uneasy partnership until they realise they are not as different as they thought.