I don't know that it is their best, but it's a top album, and really, there aren't any really "bad" Priest albums. Turbo is very different, of course, but for me, it's more about the songs. There are bad songs on certain albums (I don't care for "Epitaph"; "Evil Fantasies"; there are a couple others). I know for me, the sound improved a lot with British Steel, and personally, I know he is a pariah among fans, but I think Dave Holland was the best drummer for Priest (or at least my favorite, his "proclivities" notwithstanding).
Sorry, I don't mean to offend, but I don't count the Ripper albums.