If I had to pick one in terms of getting better in the 80s then Iron Maiden.
Metallica was superb until Master of Puppets, And Justice was good but was badly produced.
Iron Maiden though got better as they did Number of the Beast, Piece of Mind, Powerslave, Somewhere in Time and Seventh Son.
Could argue Megadeth as well actually. Killing was a raw, not great production but a very good low budget speed metal album. Peace Sells was for me anyway the best of 1986, then So Far So Good So What was an under rated album of the lot.
But Ill stick with Iron Maiden.
Judas Priest not so much, did some poor albums in Turbo and Ram It Down, poor by their standards anyway, not necessarily the public's standards.