Delenn putting the verbal smackdown on those Earthforce ships in "Severed Dreams" is my all time favorite, but here are a few for the true connaisseur:
K'Gar: Careful, Mariel, if I were married to Londo Mollari, I'd be concerned.
Mariel: G'Kar, if you were married to Londo Mollari, we'd all be...