I'd have to go with Trials and Tribbleations.
It is kind of fanservice, but it is very good fanservice.
Darmok's a good episode too, and it has all the elements that made TNG great, but it's not one of the absolute highlights of the series.
Yeah, but TaT is way better written than Flashback or Relics, and unbelievably well edited. You wouldn't think they would be able to put DS9 characters directly into TOS scenes and make it look seamless. And the meta-humor is some of the best in the series. I don't think it should be considered one of the absolute best DS9 episodes, but it's one of the better ones, and I can't imagine how they could have done better with the premise.