I just now started watching it at nineteen! I saw one episode when it first came out (I was eight) and since I didn't understand it, I never got into it. (Although I did like the "OMG, you killed Kenny!" parts a lot). But anyway, I started watching it a few months ago and I've probably seen about 3/4 of the episodes now. Although it's a tough choice, I'd say my favorite one is "Woodland Christmas Critters". I just bought a South Park Christmas DVD with that episode on it, so I'm happy.