I didn’t like Rowena and Castiel together all that much because it was a useless waste of Castiel’s time and power, but it did result in a laugh or two, like when she called him “feathers” in “Brother’s Keeper.” Still, the best line was in the episode before that.
Rowena: I’m sorry, you’re just fascinating, and angel that rejected Heaven. That’s like a fish that wants to fly or a dog that thinks its people.
A little later after Sam arrives…
Castiel: What about her?
Castiel: I’d be happy to kill her, she just called me a fish.