Eric Cartman is supposedly based off All In the Family's Archie Bunker, who has a lot in common with Cartman in terms of personality. Alternatively, he is the generic token fat friend, who, as far as anyone knows, is not based off anyone in particular.
****Edit.. I'm not sure who wrote the above. However, I have read and heard many times, that: Eric Cartman is based off of one of Trey or Matt's friends. Someone they (Trey or Matt / not sure) actually still know today, who happens to have very sociopath like tendencies.****