Season 4 started out well enough, and I definitely liked seeing Angelus so much, but that ending for Cordelia was obnoxiously stupid and bad. It was painful to watch. Really, the writing for her is just terrible after she becomes overtly bad (at least to the viewer).
Season 5 overall is very meh for me. I hated the law firm setting. I hated campy Spike that seemed to lose all of his development from Buffy season 7. I intensely hated the stupid muppets and pro wrestlers episodes. But season 5 partly redeemed itself with the saddest two episodes in the Buffyverse, and Illyria was pretty cool (even though Fred was also cool).
As for Buffy, I'm not sure what my favorite season is. There are arguments to be made for 2, 3, and 5, although season 7 also had some very strong isolated episodes. It's tough to criticize charlie's list of best episodes, though I think The Zeppo is overrated (as well as Xander generally).