I've been a huge South Park fan since S01E01 and I have one main suggestion. Please watch S03E03 "The Succubus" if you haven't already. The Lochness Monster and The Sucubbus are really great characters that I'm sure you could have a lot of fun with in the game. Not to mention Chef's parents, the eye doctor at the beginning of the episode, and the awesome slaying powers of the kids singing "The Morning After" backwards.
I have seen every episode multiple times and my one request is please don't leave this one out!!
Short list of other super funny classic stuff for your consideration -
- Volcano - "He's comin right for us!"
- Death - Stan's Grandpa wants someone to kill him
- Chickenlover - "Respect my authoritah!"
- Cow Days - "I declare shinanigans on this carnival!" and "Cartman thinks he is a Vietnamese prostitute"
- Underpants Gnomes
- Sexual Harassment Panda
- Hooked on Monkey Phonics "No phonics monkey! Stop jacking off!"
- Fingerbang (the boyband)
- NAMBLA "-Man on Internet: I'm 9 inches. -Cartman: Midgets piss me off"
- Fat Camp
- Scott Tenorman Must Die
- Proper Condom Use
- How to Eat With Your Own Butt