Oh, the PMS episode ("Bad Moon Rising") - always cracks me up. "You're on me like a monkey on a cupcake!"
Love the frog lady episode - "She ate a FLY" - especially the end where Robert accidentally causes a tray of drinks to spill over a lady in a bar, who turns out to be Amy. The audience gasp at that reveal was priceless!
But I think my favorite is "Halloween Candy", where Frank accidentally hands out condoms to the trick or treaters. Classic comedy, and Frank and Marie as Frankenstein and the Bride of Frankenstein always cracks me up!