Poll of the Week: Favorite bottle episode?

Bottle episodes – episodes where nearly all scenes are shot on existing sets – are a mainstay of science fiction television. There are few or no guest stars, the action tends to be light and special effects are not generally used, or are kept to a minimum in bottle episodes. This was most often done to save money over the course of a season to be able to spend more on major events and season finales.
Every series had them, some more than others! Many people really enjoy them for the more character driven plot lines, while others prefer the episodes were our favorite characters have away missions to exotic places or get in epic space battles.
So which was your favorite bottle episode? Head to the forums and tell us!
Bottle episodes gave us many excellent character driven episodes and there are so many to choose from. It was difficult to narrow down the list! This question comes from out General Trek category, where the polls focus on moments, characters, starships, and episodes from the various series and movies.