zopzop
Lord of the Great Abyss
It would have to be Oblivion and MAYBE, I emphasize MAYBE, Death.
Reasons for Oblivion :
He's more powerful than Galactus and the UN would ultimately do nothing to him since nullified beings are sent to him.
Reason for Death :
She's more powerful than Galactus and in one reality, where the UN was used by someone with Cosmic Awareness, everyone and everything was wiped out, EXCEPT Death who was seen laughing at the end.
Also in Marvel - The End, she was one of the last beings standing after Thanos had absorbed everything including the Living Tribunal.
Your challenge, LordofMurder, requires someone capable of a) surviving the UN and b) taking down Galactus. Off the top of my head these are the only two beings capable of the task.
You have lesser beings like Deathurge, that can probably survive the UN, but they have no way of taking down Galactus.