Definantly the 70's, with Unleashed in the East getting the most plays for me. It just brings most of the best of the era and kicks it up about 5 notches, giving it that truely metallic edge.
80's is great too, with Defenders of the Faith being a masterpiece. I actually enjoy Turbo, but think the first side is much stronger. Can't say much for anything else afterwards, although Painkiller is great and hailed by many as the greatest, it is by far my favourite. Its a ripper of an album, but its just not the classic Priest sound that I am hooked on.