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.