2. "The Paradise Syndrome"
Dr. McCoy is a skilled and highly competent surgeon, but his emotions often get the best of him. This is especially true if Captain James T. Kirk is in a situation where he is hurt and/or missing, as is the case in this episode. After losing his memory, Kirk becomes Kirok, who is believed to be a god by most of the tribe, with the exception of the jealous Salish.
This episode does provide us with a great Spock/McCoy bickering moment, though. McCoy is unhappy with Spock's decision to cut off their search for Kirk (forgive the pun) and head off an asteroid that will destroy the whole planet if the Enterprise doesn't deflect it in time.
McCoy argues that they should stay because what harm can a few more minutes do? Spock is patient yet annoyed, and then uses two rocks to explain a bit of physics that results in the asteroid hitting the planet, killing everyone, including Kirk. Bones' humbled look lets us all know who won that round.