Definitely on of the series' best episodes. After Cartman eats the skin of all the chicken Mrs. Marsh has brought home while everyone is out of the room. This pushes the rest of the boys over the edge, and they decide to just ignore him, rather than continually deride him since that doesn't seem to have much effect.
Cartman's reaction to being ignored is hilarious. Circumstances help to reinforce his belief that he's dead. After enlisting Butters' help, he decides to make amends. We learn that there is more to Cartman than just a selfish, psychopathic little boy. He proves that he can be a sympathetic character and that he is capable of contrition. He also shows that despite that, he is still manipulative and vindictive as well.
An important part of this episode is Cartman's relationship with Butters, who decides to help Cartman against his will because he is the only one who can "see" Cartman and is afraid Cartman will haunt him forever.
There were no weak points in this episode. Everything from the set-up to the song ("Make it Right") were very well done. Episodes like this are what makes South Park special.