Ooooooooohhh, there are so many!
I wanna see Lobo vs Wolverine (because according to me Lobo should wipe the floor with Wolverine - despite what happened in the fans-voting crossover - Wolverine should not be in Lobo's league... Lobo is in Superman's league, for crying out loud!)
I wanna see Conan vs Tarzan. To the finish! My money is on Tarzan though, I don't care if the Cimmerian was born on a battle field.
Black Panther vs Batman - I think they are almost exactly equal.
Same with Cap America and Spider-Man: I almost wanna say Cap will take it, simply because of his superior tactical skills, but Petey's Spider Sense might be a defense against that...
And I wanna see Flash against Superman - I know you are gonna laugh, but I still contend Flash is very under-rated!