Well fanboys drop the cap and ill give my most honest and unbiased answer.
Answer: Lucifer Morningstar
Explanation : Well here my explanation its quite simple Morningstar is just more powerful. Lucifer to begin with is a archangel and by that it means he predates creation, was responsible for creating it, and is above all concept created after creation.
Beyonder may be millions of times more powerful, however you gotta take account that Marvel cosmicology was different beings Like Living Tribunal,Eternity, Or Infinity were relatively new concept and weren't as powerful as they are now. Also its was only a claim/statement as even if it did get proven( not really, but Beyonder was greater than the multiverse) you still gotta take in marvel cosmicology a little different.
If Beyonder is millions of times more powerful then straight up i believe Morningstar is infinitely greater. Well unlike Marvel, DC already establish a good cosmicology to scale up Morningstar power as Dream said " he is the most powerful being in creation, only his creator is greater." All the endless who are the concepts of who they represent themselves are really powerul way above multiversal and if Morningstar is greater then the endless he straight up is infinitely greater than the DC multiverses.
For anyone confused on Mike Carey Logic ill explain Universe = Creation = Multiverse he describe the universe as a totality of everything hence why he said you could call it a multiverse. Creation means as a whole which meant Multiverse also. He just isn't fond of using the term Multiversal and even then Morningstar power prove he is way above multiversal.
Let me bring in a character from each side for Marvel we got the Molecule Man blasted by a weapon with Beyond realm power he was given the power to manipulate and shape matter and energy and since matter and energy is all that is then he can do anything. Well if you put it that way put it this way Morningstar has infinite Will/ Will of God which is the same thing, however unlike molecule man he needs the stuff to be there and the exact amount since he control its he can just replicates the molecules until it matches him. Lucifer on the other hand only needs a single strand of anything and can make anything from it. Like a single molecule can become a whole multiverse. Plus just know Lucifer predates molecules,energy,or matter so Molecule man can't do anything.
On DC we got Michael Demiurgos he is god's right hand and has the Infinite power of God. His Demiurgic power is so insane well its includes such concepts as Time,Dream,Cosmic,and Even Primordial Power. Not only that it also encompasses Emotion,Dream,Reality,and Life. When Elaine Belloc has the power she felt everything in creation pyshical,mental,spiritually,or even divine. Plus if molecule man came whats he gonna do the Demiurgic power itself is greater than Beyonder or him and you can't use molecule as it would fall under the category of Demiurgic power.
Feats: Beyonder erased death, has more power in a single electron than the entire galaxies, tank a explosion that could destroy several billion dimension, survived the Beyonders Bane, went up against Molecule Man, and beat the council of god heads and the cosmic entity.
Well he only erased death, but used so much power that he didn't have enough to bring the concept back unless there was a great deed/ a sacrifice being made. Also lady death or Marvel death isn't really impressive she ran form the chaos king, is weaker than thanos, has no claim over her brother or sister(Eternity,Infinity,Or Oblivion). Vertigo Death>>>>>>>>>>>>>>>>>>Marvels Death.
Lucifer Morningstar: Destroy the logos with just a touch, shattered the Mansion of Silence with his bare presence, and ripped and burned Destiny pages of his Book.
Whats so impressive with that you may ask let me explain
Logos: Direct tower that connects your voice to Gods, so breaking it would be impossible unless your just Outerversal like Morningstar
Mansion Of Silence: Has the word of god in an infinite vast are where there are mirrors to different failed universes that contains billions of soul are reflecting off of each other in a infinite timeline and infinite logic.
Destiny Books: Contains all that is of the multiverse that includes Fate,Time, Logic,Irregular, and anything you can name and has the multiverse just in the pages and Lucifer casual rips it and burned a bunch of pages.
Lucifer straight up threaten Death and the endless on multiple occasions if they were to mess with him or his universe.
Lucifer has went up against Infinite Power which is Michael Demiurgos and Michael could never put his brother down.
He tricked Gods and Two Giants( who inherited Yahweh Omnipotence after stealing it) and even weakened destroyed Fernis after he destroyed the throne of God.
Like I said Lucifer is just more powerful. He casual creates Multiverse and would be technically omnipotent well have virtually omnipotence if he didn't have his weakness creating something from nothing. In fact Michael is the few character from DC and Marvel who can do that. Plus isn't an angel "essence without shape" He is the light bringer he could just mold his energy and shape it so that would create something and that is a way around his weakness.
Plus Yahweh divided or split his omnipotence into two Michael and Lucifer and that why they are omnipotent, but the Presence changes his mind because we wanted both of them to work together so they could create a beautiful Multiverse so he gave them weakness to accomplish that and made The Great Plan that Lucifer later escape.
Lucifer can comprehend the void, is a meta where he knows his fictional,and has meet the Silk man who devours anything in his path and he has to billions of universes. Later he killed the Silk Man plus Lucifer blinked 20 billions years into the DC Multiverse history, Gave a man a heart attack from several universe away, and with the help of Elaine he and her moved the multiverse until the reached the void they carried everything physical dimensions, spatial dimensions, and everything so they moved all of creation.
Plus it was noted no one, but God can destroyed Lucifer for good.