For me it's a toss-up between when Peter and Michael Moore fart Dueling Banjoes in adjoining bathroom stalls, and the Muddy Waters passing a kidney stone.
Honorable mention, "I haven't felt this bad since God told Abraham to kill Isaac," and they cut to the Love Boat, and there's the bartender and then Lincoln comes in and shoots him. I honorably mention that one, because that's the thing that hooked me into Family Guy in the first place. :)