That was a good one, and I also personally really liked the cover for Piece of Mind.
But for me, when Powerslave came out, I was really wowed by the artwork. I was just a teenager at the time, creating artwork of my own. My style was nothing like that of Derek Riggs, but I really dug it, nonetheless. Also, I think I had a fascination with all things ancient Egypt from a fairly young age, so the album cover was cool for that reason alone, as well.
...and the thing is, I had heard Rime of the Ancient Mariner on the radio (on some early first take thing, while on the road traveling), weeks prior to going to the store to finally get the album. So finally getting to see the album cover was like WOW!