Molecule Man

Originally a weak, shy, and fearful man, Owen Reece gained his powers via a lab accident (as it so often happens in comic books) "“ while working at an atomic power plant, he opened a portal to another dimension, thus being exposed to the energy of a Cosmic Cube. Severely disfigured and with an already frail psyche, he decided to take his vengeance upon the world.

Throughout his comic book history, the Molecule Man defeated the Fantastic Four and the Avengers, despite his inability to manipulate organic matter. After dying and being reborn, he became an even bigger threat, since he became able to manipulate anything in existence.

At one time, Molecule Man was so powerful that he was second only to a Doctor Doom possessing the power of the Beyonder. And when someone is that powerful, he surely deserves a place on this list.

Sure, just as Owen Reece said, the Molecule Man is his own biggest weakness, in the sense that his frail psyche prevents him from being the ultimate power in the multiverse. Basically, he always shot himself in the foot.