I’ll go first.
Winner:
Breaking Bad, season 3 episode 10, “Fly”
Honorable mention:
Community, season 3 episode 10, “Cooperative Calligraphy”
PS: I’m having a hard time categorizing these as real bottle episodes since they don’t feel cheap or “fillery” at all, but they are at least spatially and plot wise confined.
When a show ran out of money or wanted to spend extra on other episodes they’d either do a clip show or a bottle episode. A bottle episode traditionally uses only standing sets and existing cast under contract, with no special props or costumes.
And because they’re relying almost solely on pure storytelling to carry the episode, they’re often some of the best episodes in an entire series.
They sacrifice plot, and theoretically compensate with character.