Endgame Spawn, he can erase things from existence. If I remember the comic right, he erases the entire universe and recreates it without God/Satan in a nanosecond by thinking about it, I also think he can't be harmed by anything that isn't a heaven-forged weapon.
Hellsing's Alucard, he's practically unkillable, and can apparently will himself back from non-existence.
Those two together probably could kill anything/anyone.
Though Mercer is really just Venom/Carnage like most people have said, he's not really all powerful.
But really, something that exists, a quantum singularity, for everything/everyone who is a self proclaimed badass who's unkillable, in most cases, science will be ahead of them to tell them they're not.