I enjoy any and all bottle shows dealing with Kira Nerys and her interactions with Cardassians. Duet was amazing in that regard, especially since Kira (to me) was a fairly unlikeable character in the beginning. Duet was the first time the show proved to me that they were handling her character intelligently, which cemented her as one of my favorites come the excellent Season 3 episode "Second Skin".
I would, however, if we dismiss the valuable component of significant character development, go to bat for the season 3 episode 'Civil Defense' as a contender in the poll. While nothing very major in terms of character happened in this episode, it was a great 45 minutes of entertaining storytelling, and seeing more Gul Dukat is always welcome. The humorous way the episode introduced his attraction to Kira left something to be desired - as Nana Visitor herself stated - but it was (as far as I remember) the first time we truly saw Garak and Gul Dukat go head-to-head, and that made me very excited for more focus on Garak and what part he'd played in Cardassia - and Gul Dukat's - past.
I also remember enjoying "The Doomsday Machine" episode from TOS. I think that is one of the eeriest episodes of TOS I can recall - especially since I'd encountered the Doomsday Machine in STO not long before watching the episode - which definitely enhanced my viewing of the episode as a first time viewer.
Also, yes, I love Cardassians as a species, and I enjoy bottle show episodes immensely XD