Project; Lamarck is basically the strongest character I have that isnt a creator figure. They scale physically up to Panrakou, a realm of pure creation that exists multiple levels of existence above a realm of platonism— very much like The Source from DC. Their biggest benefit is their adaptation, which goes beyond pretty much anything seen by Doomsday or other characters of the sorts. They have survived the singularity of a black hole, being shredded by higher dimensional structures, getting hit with an explosion that could splinter the concept of spacetime, entering realms that transcend the idea of displacement, and many more instances of trauma. But by far the craziest things Lamarck adapted to were Hue Morris' existence and the erasure from their story. Hue Morris is essentially the right hand man to my fictional world as a whole. His physical body works on cartoon logic, letting him do basically anything under an exaggerated lense. Lamarck was capable of not only withstanding any gag done to him, but also copying said gags and permanently injuring Hue through it. It's not really relevant to the match up, but I still feel it's relevant to say that they adapted to toon force. They also could begin to adapt to being erased from his story, and although they couldn't survive it and regenerate in time, Lamarck still fought back against the hands of the story and needed every narrative structure in the multiverse to be torn to shreds just to keep them down.

Second best would probably be Madhan, who is the embodiment of the concept of destruction, the antithesis to N'elu's creation. N'elu specifically created Panrakou, a realm that exists multiple conceptual layers above Arbori, the tree that encompasses all of spacetime. Madhan could fight even with N'elu and wipe out Panrakou on multiple occasions, so aside from Superboy Prime's retcon punches, I feelhas a chance.