Maybe Stan's mom and/or Garrison's father is the most normal on South Park (http://www.cartoon77.com/products/South-Park-Complete-Seasons-1-12-DVD-Boxset-cartoon-1494.html).
Butters absolutely not, he would jump off a bridge if someone told him to, and neither the mayor, she's so stupid that she completely forgot she had no brains when she tried to blow South Park (http://www.cartoon77.com/products/South-Park-Complete-Seasons-1-12-DVD-Boxset-cartoon-1494.html) off.

If someone else, it's gotta be Stan again.

South Park (http://www.cartoon77.com/products/South-Park-Complete-Seasons-1-12-DVD-Boxset-cartoon-1494.html)