For myself there'd be many but it's difficult to name them in isolation. For instance 'In the Pale Moonlight' is obviously going to be near to #1, but I think if you don't have the buildup and character arc to that point that it might not hit with the same sort of impact as somebody who'd watched the show to that point.

It's kind of like Game of Thrones and 'The Red Wedding'. That episode is zomg, but if you watched it in isolation with no knowledge of the character arcs to date, it'd probably be interesting, but wouldn't have anywhere near the same sort of punch.

Duet is monumental. I've watched all the recent greats, from Sopranos to The Wire, and Duet remains the best realized hour of television I've ever seen.

The Visitor (S4, E2) was nominated for a Hugo award. The main character in this story is an older version of Jake Sisko, played by Tony Todd.

"Far Beyond The Stars" is also incredibly tight and affecting writing and Avery Brooks delivers an absolutely stone cold believable performance while also keeping up the very Avery Brooks scenery chewing. Might be lost one someone who didn't know the characters outside of the Benny Russell scenarios but holy smokes.