To me it seems like Stan is the main character and Kyle is kind of a "second main character", but Cartman is like the most beliebt one...you know, when most people think of South Park the first character they think of is Cartman.
Clearly There is no main character and that's not even questionable, If Du have watched all the episodes. There are episodes Stan is the center of attention there are episodes all about Cartman, Butters even gets his own episode, I guess Du could Count which the number of episodes specifically about a character and tally them up and Du have a result. But let's take simply Imaginationland trilogy, no main character, butters is chilling in Imaginationland, Cartman trying to get Kyle to suck his balls, Kyle tries to make sense of everything going on. None Mehr important than the other, but all equally important. Matt and Trey are brilliant in that they choose which character will best help them convey there point with the characters who have the personalities best fit for that episode, if the character doesn't have a reason to be there, he's not there, There are plenty of episodes where one, two, three, sometimes all four of the usual "gang" do no not appear at all. In conclusion I just don't believe that Du can say there is a main character.
Stan in my opinion is the main character because in almost every intro, he is the first person Du see, he (and kyle) speaks before Cartman and Kenny. And he is based of Trey Parker. HOWEVER, Cartman is a VERY beliebt character and favorited Von Trey Parker. So I say Stans the Main character and Cartmans the beliebt character