The X-Men #26

Titled "Holocaust!", this November 1966 Marvel issue puts the original X-Men — Cyclops, Angel, Iceman, and Beast visible in their yellow-and-blue uniforms — on the back foot amid smoking rubble, with a commanding, elaborately costumed villain looming over them in a blaze of fire and energy. Professor X's strained face hovers at the top, and Iceman's icy trail sweeps dramatically across the foreground, giving the cover a genuine sense of overwhelming odds. With pencils by Werner Roth and Jack Kirby and inks by Dick Ayers, it's a crisp, kinetic snapshot of Silver Age Marvel mutant action at its most urgent.