@Steliyan_Petkov_Georgiev, I didn't know the usual answer, yet it seems I'm the usual respondent :). I like Cartman best because he is the one who adds so much colour to the series. He can also be perceived as the evil character we tend to sympathise with in other series (e.g. Ben in Lost, T-Bag in Prison Break, etc.).

Definitely Stan.
The usual answer to this question is Eric Cartman. I don't know why but people think it is very cool to like him.
I like Stan because he almost always does the right thing or comes to the right solution in the end. He is practical and sane.