When I think about the amount of time I poured over the "Favorite Episode" category of this challenge, the time spent on my Least Favorite episode is kind of laughable. I read the prompt, thought for approximately 2 seconds, and said "Runaway Bride."
Ah, the dreaded* Christmas Special.
And what did the Creepy Robot Santa-Mask people bring us this year?
Donna. Noble.
(pause for screams)