1. Human Nature/Family Of Blood
Interestingly enough, this story was originally a novel written for the Seventh Doctor. It would have been cool to see it played out with Sylvester McCoy, but it's hard to deny that it's the perfect story for Tennant's interpretation of the Doctor. He's always been the one who tried so hard to be human, so it's interesting to see what happens when he gets his wish. And honestly? He's kind of a dick. A product of his adopted age, he's dismissive of Martha because of her race and position, and he sanctions the beating of one of his students. He's weak and emotional, with all of the flaws of humanity. But it works well, because this is probably the episode in which the Doctor is the most reliant on his companion. Martha rises to the challenge marvelously, proving once and for all that she's a much better character than people give her credit for. Harry Lloyd does a magnificent job as the main villain of the piece, and his speech to the headmaster about World War I is incredibly chilling. There's such a tense and foreboding atmosphere, knowing that this is 1913 and in less than a year these little boys will be off fighting in France. It terms of emotionally evocative writing, this has to be right up there with the best of Doctor Who.