Most Pompous Fight: Darkseid vs. Thanos
These two would spend the entire time talking about how regal they were, and how they were feared on their homeworld, and how they would not tolerate rebellion, and such.
It would be a fairly even match, but I think it would be filled with so many word balloons it would seem like an issue of "Ultimate Spider-Man" during the Bendis years.
Most Confusing Fight to Narrate: Captain Marvel vs. Captain Marvel
Think about it: "Captain Marvel throws a punch and sends Captain Marvel flying. Cap flies into Cap, but Cap is too fast and speeds out of his way..." It would go like this indefinitely.
But then again, if you think about it, this might be a pretty cool battle. I'm talking the original Marvel Captain Marvel here, not any of the others who have held the title since his death.
Most Egotistical Fight: Lex Luthor vs. Norman Osborn
Both men consider themselves perfect. Both men consider themselves flawless leaders. Both men consider themselves victorious time and again against their enemies. And both men have that one thorn in their flesh: the main hero they can never stop.
They'd spend a lot of time bragging about how they outsmarted this hero and that, and how they won this political victory or that, and then they'd both get ugly and start screaming "Look, it's Spider-Man" and "Hey, there's Superman" at each other.
Most Evenly-Matched Fight: Scarecrow vs. Scarecrow
Let's see, both men dress up in scarecrow outfits. Both men deal with fear, and neither is a trained fighter or anything. As a matter of fact, both men are basically punching bags when the time comes for fisticuffs.
Marvel's version has been beaten down by the Falcon, a c-list hero if that. DC's Scarecrow has been beaten down by...well, just about everyone. The only thing that could possibly turn the tide to his favor would be if he gets to use his fear gas on Marvel's version. Other than that, I predict this one to be a one-punch knockout for whoever gets to throw the first punch.
Most Paranoid Fight: Rorschach vs. The Question
Both men are brutal when it comes to fighting. Neither man would hesitate to do whatever it took to win. Both men would have awesome trenchcoats and hats going for them, and even an artist with horrible drawing skills would be able to do this one.
Both men would probably spend their time spouting government conspiracies and secret society machinations. In the end I think they'd just break down and find enough common ground to go grab a cup of coffee (or a can of beans). Still, you have to admit it would be a really fun read just for the conversations!
Most Boring Fight: Watcher vs. Monitor
In this corner, Uatu the Watcher. In the other corner, the Monitor.
And then, for the next two weeks they just sit and stare at each other until the match is declared a draw.