Though not as iconic as most of the others on this list, Graviton's power set and past clashes with the Avengers in the comics makes him an intimidating opponent. A scientist gone mad with power—literally and figuratively, Graviton (if you couldn't guess from his name) can control the gravitational field around him on a massive scale, making him one of the more cataclysmic villains on this list.
In the comics, he's faced down The Avengers and The Fantastic Four, and given the Disney-Fox Merger, may do so again onscreen. He's already had an impressive outing in Chris Yost's Avengers: Earth's Mightiest Heroes cartoon series, built up to as a man so dangerous he had to be put to sleep for years to prevent his powers from hurting others.
Proving to be a charismatic and intimidating enemy, if his Yost-verse origin story—a S.H.I.E.L.D. scientist whose experiment went wrong, who has the powers of a god and a grudge against Nick Fury—is incorporated into the MCU, it would make for a hell of a story.
Especially if, like in the Yost-verse, he was part of a multi-villain prison break where even he, with all his immense power, wasn't the most dangerous villain.