I had to think about how to answer this and I thought about it in terms of "If I could watch one scene right now, which one would it be?"
For me it's the runabout scene between Jadzia and Worf in Change of Heart. Things never go right for Worf and it's one of the few times when he is relaxed and happy. Since he's my favorite Trek character, I liked that.
But I think the monologue at the end of ITPM is the best scene in DS9.