It's been too long since I've last watched DS9, but skimming though the episode summaries, I'm guessing it's s4e5 "Rejoined"
A Trill scientist (Susanna Thompson), whose host once was married to Dax's previous host, arrives on Deep Space Nine; the former lovers must decide if they want to resume their relationship and defy the laws of Trill society.
I really like DS9 but there are a lot of worthy contenders for the worst episode. The one where Sisko renders a whole planet uninhabitable just because one guy pissed him off comes to mind and of course Move Along Home.
Worfs character gets assassinated a lot in TNG and DS9, mainly him being an extremely shitty father...