The Molecule Man has been kicking around the Marvel Universe for a long time. Real name Owen Reece, he debuted in Fantastic Four #20, a bitter, timid would-be supervillain who had obtained unbelievable powers through the standard lab accident. (By the way, the accident that granted him his powers also opened the pinhole into the Beyonder's universe, triggering the decades-later events of Secret Wars). Unbelievable only begins to cover it - the Molecule Man exercised control over all molecules - matter, energy...only a self-imposed mental block prevented him from affected organic molecules. Still, with all that power, he was held back by his mental instability and inherent lack of ruthlessness.
After some psychotherapy, Reece was on the mend. He'd given up the super-villain biz, and was seeking a normal, quiet life when the Beyonder swept him off the Battleworld. It was there that Doctor Doom removed that mental block, essentially rendering the Molecule Man all but omnipotent. That 12-issue series also contained the iconic moment for that character (as it did for so many others), when he dropped a mountain range on the battered heroes, leading to that classic tableau of Hulk holding up billions of tons of rock. Good stuff.
In the end, Molecule Man proves able to rescue the villains and head home across the galaxy under his own steam, with new-found ladyfriend Volcana on his arm. He was still sort of a wimp, but he had begun to develop some real confidence, and even handled such rough characters as the Wrecker and Doctor Octopus with relative ease. He'd come into his own. As the unfortunate cash grab of Secret Wars II unfolded, he played an even more central role, as the confidant and later vanquisher of the Beyonder. Still, I like to think of him as the guy dropping mountains on people and reigniting stars. Plus, he made lightning-shaped facial scars a fashion statement long before that boy wizard did.