The Cybermen 2-Parter. Are we really supposed to find these tin robots at all menacing? Or believable? They can build these fantastic cybernetic bodies, but they need human brains to power them? How... 60s. (I understand the initial idea was to update the concept to something altogether more cyberpunk-y -- I wish they had.)

The whole "mirror-Earth" concept is simultaneously tired (yawn... Rose's dead, loser father is neither dead nor a loser) and underexplored (wouldn't there be a parallel Doctor in this universe?)