I gonna go with Ma hunkel the Red tornado!
There’s one in every crowd…
In a non-cosmic fight, the Martian Manhunter would do insane damage. He’d make you believe he was your mother, then compel you to kill yourself. Or other people, then yourself.
Also, the Vision. No emotions, he can control his mass, has flight, and superstrength. Whizz, whizz, bang, splat goes most of the Marvel and Dc universes, anyway.
I think the Saint’s powers are framed in absolutes: He never misses, and anything he hits, he kills. It may seem absurd to state that he could hit a being less than a planck length tall with a bullet, but he could.
Which is why I don’t like to consider folks like him as “Superheroes”. Doesn’t mean that he can’t be a good character for certain types of stories, but there’s no point in inviting a character with the power “I win, you lose” to a smackdown tournament, you know?
Then defeating him is relatively simple. One of the characters, Professor X for example, who has the power to Mind F*ck need only cause him to turn his gun on himself. If mutant psychic powers won’t do it, then one of the magic-based mind controllers.
The Fat Fury would own them all. Not even pull the sucker out of his mouth.
I’ve got my money on The Molecule Man, just because a super-dude whose idea of a fun time is watching “F-Troop” reruns with his girlfriend tickles the bejeebers out of me. 
You’d think, except the powerering up of a Saiyan is so impressive that, and history backs this up, everyone will sit there staring with their mouths open, no matter how long it takes for them to reach full power.
That’s going to work so well against Dr Manhattan: “Oh, I see. Yes, that is a brilliant plan and you would have succeeded in killing me three days from now. So I’m going to transmute you into a rock now before you have a chance to get started on it.”
Saint of Killers loses vs. Dr. Manhattan, at least.
White tagged spoiler.
He shoots Cassidy and Cassidy doesnt die because, as a vampire, having an enormous hole in his midsection doesnt kill him. Dr. Manhattan can survive being completely disintegrated, so I assume having the SoK blow his head off would be more of a setback than anything else. Granted, the SoK would probably just keep blowing his head off every time he reformed, but when DM eventually got the drop on him hed teleport him to Mars or some other place he couldnt return from, effectively winning.
Here’s something else I hadn’t thought of:
No matter who fights who (and my vote goes for Rogue, because if she can hang onto her absorbed powers she can just tag all the cosmic-level superheros and wipe out everyone) it’s fairly clear Death will be the last one standing.
But wait. Both Marvel and DC have Death characters. There’s still one matchup left.
Who wins then?
Another reason Saint of Killers can’t beat Dr. Manhattan: Dr. M can make multiple copies of himself. So while Saint of Killers is blowing the head off one of them, the other could transmute him into a puddle of goo.
Of course, if we’re going to include cosmic abstracts like Death or Eternity, this thing is going to get ridiculous pretty fast. They aren’t really heroes, though.
I’d say the best bet would be someone who has the power to copy other people’s powers. Like Composite Man, who can copy the powers of anyone he’s ever met. (He’s a villain though, not a hero.)
But see, that’s the thing. If Dr M knows that Batman will kill him in three days then there’s nothing Dr M can do about it. He knows what others will do and he knows how he will react to it but he can’t change how he’s going to react based on knowing what the other person will do or knowing how he will react.
I disagree. It’s difficult to imagine how a true omniscient viewpoint would work but I don’t think it requires the passivity that you’re surmising.
Dr Manhattan can still choose to fight against Batman, the same way that Superman could. And like Superman, Dr Manhattan’s very likely to win because of his superior powers. The difference is that in Manhattan’s case he already knows he’s going to win before the fight starts. To use the example I gave, Dr Manhattan already knows today that he’s going to kill Batman next week because if he doesn’t do that, Batman will kill him the week after that.
I don’t have my copy of Watchmen in front of me to look up the pertinent dialog; but, at one point Manhattan makes a statement to the effect that he is also a puppet, albeit one who can see the strings.
Yeah, it is shown in Watchmen that Dr. Manhattan is basically a slave to his foreknowledge – e.g., he does things simply because “that’s what I do then”. For instance, when someone “surprises” him with evidence that his powers may be giving his friends cancer, he gets upset, even though he of course already knew they were going to spring that on him.
That said, there are certainly times when he acts on the basis of his foreknowledge – just, only when he is already predestined to do so. From Dr. Manhattan’s perspective, the future is already written – meaning if Batman is predestined to defeat him, he will, and if he’s predestined not to defeat him, he won’t. But I think Dr. Manhattan winning the fight is a lot more likely, if for no other reason than that there are a lot more scenarios by which I could see him winning than losing. So, odds are that one of those will be his predetermined destiny.
I saw Ambush Bug.
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He would teleport away from any danger and let the rest kill each other.
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If he did happen to get killed, he’d just sneak out the back door of Hell and rejoin the battle.
And the 2000AD universe remember has Judge Death, so that’s a three way match 
Or maybe a two vs six if you count all the Dark Judges and the Sisters of Death 
Ah, but I’d argue they’re not real incarnations of Death, just undead creatures with really freaky supernatural powers. They’d still be within the reach of normal superheroes–don’t forget that Batman captured Judge Death at one point.
Still comes down to Marvel Death vs. DC Death. Personally I’d like to think they mop up, have a cup of tea, and go home, but still–who’d win?
I’d have to go with Dr. Strange. Excluding cosmics (like the Endless or the Celestials etc.) Strange is just too much. He’s gone toe-to-toe with beings at least equal if not greater than Nabu. He stood up to Adam Warlock when he had the infinity gauntlet and survived!
In the comic battle boards the Silver Surfer has feats that trump anything that Dr. Manhattan does.
Also, in fights like these, speed kills. Without automatic shields guys like the Flash and the Silver Surfer are killing half the participants before most of them can utter a word. Kind of hard to call on Johnny Thunder when your head has been torn off.