So, it's a horrible monstrosity in the shape of a dead man that devours the flesh and brains of humans and spreads plague in it's footsteps. A zombie.
But I'm willing to concede Tiki Snakes has a point. Even if ZEUS isn't really Mercer, he's still close enough to human in his emotions that it might be unfair to include him directly.
That said, the lesser monstrosities his plague spawns are more than fit for competition. The baseline Blacklight Zombie is among the faster breeds, but otherwise nothing special and not immune to non-headshot kills. On the other hand, the more advanced Hunter-types are gog-darn terrifying. Besides immense toughness and speed, they can climb skyscrapers and survive falls from the same, regenerate and always, always come in packs. The Alpha Hunter is nearly as tough as Mercer himself and, thanks to immense regenerative capability, can only be killed if it's devoured by something with digestive strength equal to Mercer's. Even repeat fire from a tank won't stop it otherwise.