Yep Butters. Butters is probably my favorite cartoon character of all time. He replaced Bobby Hill. I'm not a huge King of the Hill fan, but Bobby was fantastic. Butters is beyond fantastic though. Everything he does makes me laugh. Tweek would be next on my list as far as South Park only.
Randy Marsh. Hands down. Every episode that he is the focus of is hilarious. This last season was kind of lame but the episode where Randy is obsessed with Food Network was one of the funniest I've seen in years.
You see that Pork Loin? He's gonna sear the s*#t out of it, Stan.