^ "In Purgatory's Shadow". It's one of my favourites!! ^_^
Actually, there are several instances where Worf is in command of the Defiant.
For example, "Rules of Engagement" where he is on trial for a command decision that he has made.
Also, in "Apocalypse Rising" when Kira says that she's in charge of the station while Sisko is gone, Worf responds with "But I command the Defiant." *after which of course Kira pulls rank on him. But she is on the Defiant (just before) "Rules of Engagement" and although Kira is technically Worf's superior, Worf is in the captain's chair of course.
Sorry if this has been said before I'm just going off more recent posts here.
When in doubt, hug your teddy.