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 plot / 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.