23. The Tenth Doctor's Hearts Are Beating Out A Samba
Matt Smith got to heroically save the world from the Atraxi in his first episode. David Tennant got to do... this. Bear in mind that the Tenth Doctor had spent most of his actual first episode confined to a bed, so fans were expecting bigger and better things when he finally returned with some sort of vengeance in his first full series the following spring. Alas, instead they had to put up with him getting possessed and acting like a right old fool in the name of "comedy". At least they got it of the way sooner rather than later.