There are several South Park moment that I enjoy but I don't have a personal favorite, most of the parts always make me laugh slightly, others sometimes make me laugh more. One of my favorite moments was in the episode with Bono, but I'm not going to state it here since I believe it's against the ToS.
One of my favorite moment is when Stan's dad is trying to make the biggest crap ever. Near the end, his poo send him up from the toilet around 5-7 feet. And when the poo is going up it spins, so Stan's dad kind of just spins around on top of a column of poo.
Cartman explaining how he managed to kill Scott Tenorman's parents and trick him into eating them, followed by Cartman licking his face and drinking his "tears of unfathomable sadness" is easily my favorite scene. That was the moment when we realized that Cartman can literally accomplish anything.