One of my favorites is 5X02, "The Ship."
I also love the Circle trilogy. Talk about a coup, landing Louise Fletcher (Winn) and Frank Langella (Jaro) for recurring roles certainly was. Not the regular cast wasn't good as well, but those two brought something special to their scenes, both separately and together.
Also, "Treachery, Faith and the Great River," two Weyouns for the price of one, LOL!
"Crossover," solely because it introduced sexy Intendant Kira! WOO!