Trials and Tribble-ations is very high on my list of favorites. It's a whole lot of fun, and i love how Odo responds to the tribbles. As well, the episode is a tribute to TOS that's fond and respectful without going over the top or being cloying (as I think it could have been in lesser hands.)
I was just talking about Trials and Tribble-ations this morning to a Trekkie friend who hasn't seen very much DS9. He shocked me when he said he's never seen it. How could any self-respecting Trek fan miss such a magnificent treat? Needless to say, he's going to be getting my copy of Season 5 as soon as possible!