X-Men (1963) #31
First appearance of Angel's girlfriend, Candy Sothern, and the Cobalt Man. Written by Roy Thomas and penciled by Werner Roth. What if the X-Men took on someone with the powers of Iron Man? When former Stark Technologies employee Ralph Roberts shows his brother Ted and Jean Grey around his new privately owned lab, he reveals a darker side of his personality, one filled with murderous envy.
A prior bump on the head combines with the power he gains when wearing his cobalt armor (copied from Stark's own "bodyguard" Iron Man) to create the Cobalt Man, a super-menace for the X-Men to contend with. Professor X's merry band of mutants can't afford to take their time reeling him in either. The Cobalt Man has become a walking nuclear time bomb!
The Cobalt Man returned years later to menace first the incredible Hulk (issues #173 and #174) and then the rest of the Defenders (issue #43). Also, check out this issue for a resolution of the Cyclops - Marvel Girl - Angel love triangle. Here's a hint: it involves the reintroduction of a sultry brunette from Warren (Angel) Worthington III's past -- Candy Sothern. There's also a finger-snapping peek at the Coffee A Go-Go where beatnik poets spout free verse for a cup of espresso. Like wild, dad!