Well if you treat the celestials as a race of alien space gods.... Seeing that as a race they come to f with marvel earth all the time...
__________________ Quotes from Hia8:
"I claimed that the science is sometimes faulty."
"You don't understand. This is fiction. That means none of this stuff really happened."
"There is no writer to purposely ignore a character's natural ability just because it suits the story."
"in some cases because the writer knows that Character A will dominate Character B easily and refuses to allow this to happen for the sake of the story."
Not sure if Spectre counts towards this since his origins are heavily tied into Earth, and Synnar when he was a big deal as well
So far the biggest threats off the top of my head would probably be Imperiex, AM, and Mandrakk
Then the three biggest for Marvel would be off the top of my head again Gayos King and his band of queers, the Infinites, and some other random dickhead
Also this battle is far too open. You have pretty much every race in Marvel going against Marvel Earth, and you have a bunch of omnipotent beings on both sides
There was a short arc in Superman where Mxy turned evil and started attacking the planet. Canceling out gravity and whatnot. Mxy twins. He almost killed someone as well.