Batman vs. Lex Luthor: Suit or no suit, Batman knows how to put him away.
Green Lantern vs. Captain America: As much as Captain America is a super soldier, I have to go with GL.
Spider-Man vs. Thing: Spidey. The guy has fought against tougher guys and won. Thing? No big.
Spawn vs. Wolverine: As much as I'd like to say Wolverine, I say Spawn can take him on, and destroy his adamantium skeleton, too. All you need is the right amount of heat and boom! Bye-bye Wolverine.
Superman vs.
The Flash: I don't know. I'm just picking Flash for the hell of it.
Juggernaut vs.
The Hulk: Half powered Juggernaut or not, he doesn't get any stronger the more pissed he gets. Hulk, hands down.
Magneto vs. Ultimate Thor: We're still not sure if he is indeed a God, (though I say he is), but Magneto can take him on. If Thor brings the hammer, he can somehow make Thor loose it (it's been done plenty of times in 616) and finish Thor there.
Earth-2 Superman vs. Superboy Prime: Again, don't know. Just picking anyone.