darkSpyro

Bill is a chaos god that uses his charisma to get into the minds of others until he becomes the most powerful being in the universe. At his peak, he had the power to break through dimensional barriers, create his own mini-dimensions via "Time Bubbles", morph to absolutely gigantic size, shift objects to his liking, had an army of Eye-bats that could transform anything into stone, the ability to change his appearance, and much more. Had it not been for his own cockiness, there was basically only one way to beat him outright: banishing him by bringing the symbols together to create the Cipher Wheel. Even then, Bill stopped that pretty easily. He was only stopped by the Stans tricking him into deleting his existence, but that itself still couldn't destroy Bill as Stan's memory along with Bill soon came back, though he can't do much anymore.

Sans is a skeleton with knowledge that his world is in some sort of loop. Whenever you reset the game to play again, all of the characters lives reset and their memories are erased. Except for the player. Somehow, Sans found out about all of this. You find out about this through the Genocide Route, where he exists as the final boss and easily the hardest enemy in the game. He has the ability to use weapons called Gaster Blasters that can destroy the player almost instantly if used in succession, send bones out at terrifying rates, and manipulate the RPG mechanics that the game is based around such as morphing the attack screen, attacking you in the menus, and being the only enemy that can (and always will) dodge attacks. His final "special attack" involves him not doing anything as he knows if he does than eventually he'll slip up and get killed, so neither the player or Sans can do anything. The negative portion of Sans is that he only has a measly 1 Attack stat and 1 HP, making him technically the weakest enemy you face. Adding to that, the knowledge that his life will be reset no matter what he does has caused him to not try, so he gets lazy. His laziness is his downfall in his fight, as he falls asleep while using his "special attack". This gives the player time to nudge the attack screen over to the FIGHT button and destroy him with one hit.

If Sans were able to find out information about how to defeat Bill, he might have a bit of a shot. Otherwise, Bill Cipher destroys Sans completely. I'd say 9/10 chance of Bill Cipher winning.