Galactus

Since we’ve mentioned Doctor Doom, we should mention Galactus, who isn't quite a villain, although he acts as though he’s a powerful antagonist. Imbued with the Power Cosmic, he’s basically the personification of the entire Cosmos. He’s literally a god, and he feeds on entire planets to survive. The fact that he’s above the morality of mere mortals makes him invincible.

He possesses a vast array of powers and abilities, and he could be the next big threat to the MCU superheroes. With his rather tragic background (he doesn't really want to destroy civilizations, but he has to), he'd make an awesome antagonist.

Along with Doctor Doom and Silver Surfer, Fox owns Galactus, and they transformed this tremendous superbeing into a simple cloud that swallowed Earth. Go figure.