The Visitor seems a pretty popular choice but I wasn't really fussed on it. I didn't hate it but I wasn't overly impressed. But I did watch it out of order so I was thinking "what about the war with the dominion?" and "Jadzia lives to be old" so to me it seemed like saving Sisko was murdering a bunch of other ppl including Jadzia. Pretty sure that's not what I was supposed to be thinking during the episode. But even without that it didn't really make me sad because Jake and Captain Sisko have never been my favourites.
I really enjoyed Trials and Tribble-ations and its the sort of episode that is fun to re-watch. Most of the mirror universe episodes are good for a giggle too.