JakeTheBank
Return of the King
Spider-Man versus Superman: In a comic, it would probably be something similar to how they already "fought".
Wolverine versus Wonder Woman: Logan would probably do a helluva lot better than he should in reality. He'd probably land some glancing slashes, piss Diana off, and wind up lasso'd and incapacitated. He'd probably shrug off some of her blows, though. I wouldn't be surprised if some PIS came up with Logan's animal side somehow "resisting" the lasso, though.
X-Men versus JSA: JSA should win in a comic through sheer power, but the X-Men would probably have better tactics and strategy.
Thor vs. Darkseid: Thor would probably get his ass kicked the first time by Darkseid, but once a rematch came up, he'd probably have some epic Thor-speech as he blocks the OE with Mjolnir and slams Darkseid's face in. Just as Thor looked as if he was about to win, random plot device or whatever ends the fight for whatever reason.
Magneto versus JLA: Magneto would probably get amped or be portrayed at an absurd level for him to challenge the JLA. I'd wager he'd do better than Polaris just for the fact he's Magneto.
Carnage vs Batman, Robin, Batgirl: Honestly, probably get his ass kicked by Batman. The sidekicks would get maimed some, but Batman would deduce his weakness and exploit the hell out of them.
Thanos versus Spectre: Thanos gets his ass kicked, but tanks a lot of punishment while doing that stupid Thanos grin he does so well. Probably a lot of dialogue more so than fighting, with Thanos trying to mindphuck Spectre while debating things such as morality and how he should be above God's wrath or something.