The Planet Eater Galactus
The Devourer of Worlds is probably one of the strongest villains in the Marvel Universe merely because he eats entire worlds for breakfast.
The Likely God
The Beyonder has abilities that rival event the infinity stones where he can create and destroy matter to his will which makes him a formidable foe.
The Mad Single Father
The Mad Titan is obviously one of the most powerful beings in the Marvel universe as he has held all the Infinity Stones at a time.
The God of the Symbiotes, Knull, wields the Necrosword called All Black and is definitely one of the most overpowered entities in the Marvel Universe.
Kang the Conqueror
The time-traveling multiversal villain is definitely not to be trifled with considering he literally has power over time.
The Doctor Who's Also A King
Doctor Doom is not only one of the most menacing but also one of the smartest villains in the MCU. He is also the ruler of Latveria.