first you have to remember that after cartman finds out who his father is he say "HE'S MY DAD!" which sounds as if he is speaking of someone far removed from him and not someone he has seen a lot of take this into consideration lets say cartmans father turn out to be mr. garison then you would expect cartman to say somthing like "MR.GARISON IS MY DAD!" or if it was chef "CHEF IS MY DAD!" so in conclusion I feel that the person who fathered cartman must be kevin since his name has not been used enough to be recognizable.
P.S. this also explains why the denver broco's canot be his dad since then he would have said "THEY ARE MY DAD!".
P.P.S. but it would be cool if john elway was cartmans father.