"Definitely Galactus.
At the sight of the Silver Surfer (Galactus's Herald) the skulls made an order to shut off their whole system.
Now just pause for a second and think, a single being caused five to eight planets to shutdown . Every single thing on each planet stopped , just to let the herald pass so Galactus wouldn't become aware that they even existed.
That my friends is TRUE fear."
Big G is easily the most feared being in the galaxy, but he's not a villain. He can't help himself.
"No extras (UNLEES is one of the main things to the character, and not is not that, let me give another Clue he was good and turn evil"
Darth Vader was the most feared villain in HIS universe. He's above street level in power but below herald, so most wouldn't be terribly afraid of him in a couple other places.
Now Thanos...even other villains fear and respect The Mad Titan. People either freeze out of fear or instantly become more agreeable when he shows up. A lot like Darth Vader but much, much more powerful.