6x02 The One Where Ross Hugs Rachel [4th or so watch]
Live at the Apollo
9x02 Episode 2
13x05 Curtain: Poirot's Last Case [series finale]
A great final episode, and a great final story actually -- why isn't it one of the best-known ones? Maybe fans are too concerned with spoiling the twist (and rightly so), which has been the fate of Murder on the Orient Express.
[Watch it (again) on ITV Player.]