i came up with this question after listening a lot to 70s priest and i discovered that all(except for rocka rolla, which i haven't listened to) are quite brilliant! i didn't include unleashed in the east obviously cuz it's a live album.
a short 2 cents on each of the albums:
-rocka rolla: haven't listened to it.
-sad wings: now here's a lil bit of real metal. kind of depressive album, lyricswise, but the music will get you over, since tyrant, genocide, the ripper, victim of changes are headbanging delights and then there's dreamer deceiver/deceiver which is a really good moody song. 9/10
-sin after sin: slightly heavier than sad wings, with the thrashy intro to diamonds and rust, and what i believe to be the first thrash metal song, dissident agressor. sinner also deserves mentioning, and the two good ballads: here come the tears and last rose of summer. 9/10
-stained class: heavier still but less consistent, we get a sample of what would become iron maiden in the title track, and the album intro of exciter is a moment of pure thrashiness, and we have beyond the realms of death, saints in hell as stand-outs. 8/10
-killing machine(hell bent for leather): not heavier than stained class but still a good metal release. not too consistent, with rock forever, hell bent, killing machine, running wild and evil fantasies as the good songs. 8/10.
so for me it's a tie between sad wings and sin after sin, and i choose sad wings, because of the "ahead of it's day" factor.
and you?