SWoD is clearly the best Priest album and I'd argue the greatest traditional heavy metal album period. Every track is great, without exception.
Painkiller is a worthy second in my eyes, but obviously it's a totally different style and more superficial by nature. Of their older stuff, Sin After Sin would be in the discussion if it weren't for "Last Rose of Summer" but even then, the first two songs on side B are strong but not exceptional.
As for Stained Class, the opening four tracks aee masterful, but after that, there's a drop in quality. Invaders, Savage, and Heroes End are all solid, but are nothing exceptional.