Screaming for Vengeance has some of my favourite songs, but some of the band's WORST performances (i'm looking at YOU halford)
Painkiller has their best on-record performances bar none, but alot of the songs are filler save for Touch of Evil and the title track.
So, based on performance and songs, i'd say the mighty BRITISH STEEL!!!
This is obviously the wisest choice as it's the album that gets talked about the most in the press, pubs, garages, and the internet, but it's such an iconic album and one that put Priest back on the map. And i'm not just talking about BTL and LAM, there's some truly amazingly awesome songs on British Steel.