Deferenz- Although that is classic Cartman moment I think the 'Scot Tenorman must die' episode is his finest half hour.I espeicially love the bit where he's on the way to the 'pube fair' believing he can sell Tenorman's pubes for a million dollars, the way he's laughing on the bus journey 'Whoa ho ho! One million dollars!' kills me.
You've also got the AWESOME-O episode and Casa Bonita as the two other great Cartman based episodes.
Mr Beard-I'm don't think its a full return to form, I think the breast cancer show was close to being classic, other than that the episodes in the latter half of the run have been a bit gimmicky, although still funny. The best South Park Seasons in my opinion are 5 to 9, but even then every season has its classics.
Its kinda weird when you think about the way that the best Simpsons Seasons are 4-9 too.I guess its the natural curve of getting to know you're own characters.