Who would win between the the two most powerful characters in DC and Marvel VS the respected Universes???
Does Thanos have the Infinity Gauntlet? If so I think they win this fight. As long as they kill Adam Warlock and any other Skyfather level threats. It's a maybe, if Thanos doesn't have it... then they will be raped. And is One-Above-All apart of this fight?
Acutally the question is do they have prep.
I see it like this.
Thanos and Darkseid make a deal, They utterly destory both universes and then with the Anti-life equation dominate the next one.
Against everyone though even with IG Darkseid and Thanos will eventually lose. Since like in all Marvel stuff Thanos eventually will betray Darkseid and himself on a subconcious level.