Far Beyond the Stars.
It's just an awesome episode. It's a period piece, a love letter to SF, a character study, a discussion of racism and sexism, fan service without being pandering - all those actors without their alien makeup! I had completely forgotten this episode in favor of the arc episodes about the Dominion and anything with Garrick but by season 6 it is so obvious that the cast and crew are really comfortable, willing to take chances to tell good stories, and more often than not it pays off. Even the bad episodes are amusing just because the actors know their roles so well.