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 specially and plot wise confined.
So many good ones. They’re often done to save budget, but if written well they can be some of the best in their series.
First thing that popped into my head though was Twilight Zone, “Five Characters in Search of an Exit”.