The NAMBLA episode is also one of my favorites, definitely top 5. Other favorites would have to include "Mega-Streisand", the Janet Reno episode(they got it out three days after her stormtroopers snatched Elian Gonzales), Timmy and Jimmy joining the Crips was also a good one, but I'd have to say my favorite was the Christopher Reeves stem cell episode. That one had me in tears it was so funny.
One of my favorite things South Park did was on one season's Halloween episode, they outlined the lens with images of Streisand because she was so ugly and scary. Is this the same as the "Mega-Streisand?"