It is not only my favorite AtS season, but one of the best in the Whedonverse.
First, it has "Not Fade Away," which is one of the best episodes of TV I have ever seen.
Spike brings an amazing dimension to the show. My least favorite character in the Whedonverse, Connor, is thankfully gone for most of the season, and when he comes back, I actually like him. Fred/Illyria is amazing. Harmony, someone I never really cared for, is brilliant. Everything worked in Season 5.
The only thing I missed was Liliah. I loved her, and was sad that she was gone.