Zapp Brannigan and Kif are a package deal. Brannigan's a lot funnier when he has a captive underling to torture ("Have the boy lay out my formal shorts") and Kif is funnier when he's suffering.
I like how Zoidberg developed a personality. When the show started, he was basically there for one gag: He's a doctor, but doesn't know human anatomy, lolz. He turned into this really awesome insecure, eager-to-please failure at life. He's like a crustacean Matlock!