This is an early one, but one of my top 5 is "The Busboy". On 2 occasions during the episode, the busboys uses borderline profanity.
Kramer has a funny line when he asks the busboy: "Como se dice, waterbed?"
It's also funny that the busboy gets a brand new, higher paying job near the end, yet he takes time to stop by Jerry's apartment and states that he's already very late for his first day at work.