You want The Ultimate Showdown of Ulimate Destiny?
Yeah, there's a slew of characters such as Goku and Superman who would mop the floor with a huge margin of characters. But the problem with that is they all have weaknesses or loopholes to supplant their powers. They'll just be extremely resilient at the very least. Probably coming back death after death to only serve as a nuisance to more omnipotent characters.
Think outside the box. Surely, not a lot of our culturally-relevant figures have the best outcome compared to some. Bring in years worth of literature, theater, as well as epoches from early stages of certain mediums. The Old Ones of H.P. Lovecraft fame, Doc Savage, etc. Might as well throw in pantheons of myriads of religions there. Hell, imagine all the alternate universe interpretations of God, Satan, Thor, etc. Even some of the heroes of ancient yore who in many ways paved the way for just about every hero. Throw in Gilgamesh, Beowulf, Odysseus, Hercules! The blood from such an armeggedon's so large that it would probably wash away human imagination!
Oh, but cradle yourselves tight, my friends. I haven't even gone over the worst part. We're accounting all human imagination here. Insurmountable swaths of which we don't know exist. Surely, there's something spurred off one's imagination that would be ludicrously powerful enough to be nigh unkillable. Multiply that by the human population throughout the dawn of writing! But who do I speak of? Well, you can find them just a few clicks away on your Internet browser. They're poorly-developed chimeras stitched together into one entity. One entity explicably so powerful that the universe aligns with them on their every intent. Touted as 'original' by the bellicose teenaged gits who spawn them into thought! I speak of the tide of "Original Fan Characters." Do not steal!
So under all this, get ready for all your beloved characters to meet their banes. By the hands of Darkflame the Hedgefox, Alicorn Princess Sunshine Twinkle, and the 666th Hogake, Vegokutrunks in Super Saiyan x9000, wielding 3 Ultima Keyblades!