I can't think of many good ones. Usually when a series ends it's way past its prime. Friends had a pretty decent one. Arrested Development was a good finale in the sense that the episode was still very funny and stayed true to the show, it just sucks that it ended so soon.

As for bad, there are many. Seinfeld & BSG were some of the worst for being such good shows. Well, the first half of the BSG finale rocked but the second half was all wtf and not in the good way. Seinfeld was probably the worst finale ever though.