That's a hard one. The first one I bought was Stained Class. Sound wasn't great (probably technology at the time), but I did like Beyond..., Exciter and White heat...
Next one (for me) was British Steel. To this day, I have not found anything I do not like with this album. My first favourite was Steeler, but later I think that You don't have to be old to be wise has become my favorite. In the 80's I bought a bootleg (Unchained) - sorry - sound sucked, but it really made me thinking. Point of Entry isn't that bad.
Anyway, the answer: my all time favourites are Screaming for Vengeance, Point of Entry and Defenders.... because of these songs:
Heading out to the highway
Rock hard ride free
Night comes down
I liked Painkiller + all the "hard stuff", but the core of Priest -for me - was the above.