The Mechanical Monsters
Admittedly, I jumped the gun on that one. When I first saw the cover, I thought they'd appear inside (y'know, because that's what covers are supposed to do). When I found out they didn't, I had already included them and didn't want to take them out. It was a huge bummer. Thankfully, Rebirth has fixed this! (and so much more from what I here.
At the beginning of Superman #1 (June 2016), we get a montage of Superman's past. And right up top, there he is, punching out a Mechanical Monster. Good job, Superman! And good job, Peter J. Tomasi!