Thanos
Iron Man #55 · 1973
The comic book villain who became a household name overnight
Not evil. Hungry.
Editorial estimates on a 0–100 scale, calibrated against every other character in the index. Not official canon.
Galactus eats planets because that is what he is, the way a storm is not cruel for making landfall. He is one of the great conceptual leaps in comics: an antagonist with no malice, no negotiation and no crime, whose defeat requires convincing him rather than beating him. Kirby's design, a figure the size of a skyline in a purple helmet, still looks like nothing else.
His arrival is an extinction event, and stopping him requires a weapon that could unmake the universe.
He honors bargains and can be reasoned with, which makes him the rare villain talked into leaving.
He eats planets to survive and requires universe-ending weaponry to inconvenience. Nothing else on this list operates as a routine extinction event.
He is not malicious, which is why he ranks here instead of first, but his arrival is an extinction event for whatever planet he reaches.
A villain with no malice and no crime, who has to be argued with rather than beaten. That was a genuine conceptual leap in 1966.
Iron Man #55 · 1973
The comic book villain who became a household name overnight
Action Comics #23 · 1940
Humanity's greatest mind, ruined by one man who can fly