Most episodes feature Cartman or involve him in any way.
Seriously, episodes like Dead Celebrities and The Poor Kid would be better if he didn't even show up.
Randy is an asshole who doesn't deserve to have so much screen time
Since the fucking Tegridy farms arc, he's just a sociopathic idiot and he doesn't deserve that much prominence.
They did Kyle dirty
The poor boy often experience bullying from Cartman and is constantly used as a plot device in the episodes starring the fatass, but he remains the best character.
The female characters are unbearable
Aside from Wendy and Nichole, most of the girls in this show are shallow and are only defined as being frivolous gossips or have a drastic personality change depending on the writing (e.g: Heidi Turner) but the adult women are worse, Liane is a horrible mother and Sheila's personality is extremely inconsistent and at times domineering, not to mention characters that only appear in one episode and then are quickly forgotten (example: Rebecca Cotswolds and Estella).
Kenny doesn't even seem to be one of the protagonists
Sorry but although I don't hate Kenny anymore, he really doesn't show up much and doesn't have much personality besides being perverted and dying all day long, the creators tried to make him more prominent with the whole Mysterion thing and even created a sister called Karen for him to protect but both Mysterion and Karen don't appear much in the series, so Kenny is generally sidelined, not to mention that on the Vaccination special, he literally behaves like an innocent and fragile little kid in contrast to his other appearance where he was a lot braver and more mature.