Piece of Mind, and it's not close, but it's not really a good comparison (I get the "4th album" comparison and all, but otherwise it doesn't make much sense).
You've got VERY different time frames (Vol. 4 in 1972, Stained Class in 1978, and Piece of Mind in 1983). Sabbath was at the height of its early popularity and so strung out on cocaine that they could barely get the album recorded. Priest hadn't achieved any real measure of success yet, and Maiden was coming off the success of TNotB.
In my opinion, the only average song on Piece is Still Life. Everything else ranges from above average to all-time great. Stained Class has one all-time great song (Realms) and a couple other strong songs. Volume 4 has only a couple songs that I find memorable.