Hmm.. Quinessential trek, eh? And you've got Distant Origins on there? Isn't that the Voyager episode with the dinosaurs? Or do I have the names messed up. There's so many series! >_<
I dunno.. i'd have to go with probably a DS9 ep. Something like their series finale. I think the perfect Trek episode has elements of exploring, and learning about different alien cultures other than our own, as well as the sucessfull cooperation with all of the human race, as well as other species. Because, that's what Star Trek is. The thought that one day, everything will be peace. And that we'll be able to get along with not only ourselves, but with other aliens in this big galaxy of ours.
Of course that's exactly the oposite feeling that the producers of DS9 wanted to have.