The X-Men #27

The X-Men find themselves in serious trouble on this December 1966 cover, as a winged figure in a gold-and-red costume dominates the scene, soaring above a battered Cyclops, Marvel Girl, and Beast amid a shower of shattered debris — with Professor X's intense, lightning-framed face looming overhead. The story title says it all: "Re-Enter: The Mimic!" promises a confrontation that has the whole team on the back foot. Cover pencils by Werner Roth and inks by Dick Ayers give the scene a kinetic urgency that makes this Roy Thomas–scripted issue hard to put down.