One of the biggest hypothetical heavyweight fights between a pair of Superman. In a crossover comic, the two actually fought - where even Marvel agreed that Superman would be the God of Thunder.
In the 2003 Marvel/DC crossover comic JLA/Avengers by Kurt Busiek and George Perez, the two comic universes collided as the heroes are sent on a collectathon to obtain the most powerful items in their respective universes to save everything. In the series second issue, the Cosmic Cube is up for grabs - and Thor and Superman duke it out. The two exchange a series of blows, but when Thor swung Mjolnir at Superman, Kal-El grabbed the hammer and whipped it back at him and delivered a devastating blow. Superman says Thor may be the "single toughest opponent" he's ever faced.
So how did two different publishers decide Superman could beat Thor in a fight? With a vote. Current Marvel Senior Vice President of Publishing and Executive Editor Tom Brevoort discussed the crossover on his blog. Brevoort revealed that Superman's victory in the fight was decided by him, Busiek, George Perez, and DC editor Dan Raspler. He said that each of them believed Thor would lose the fight.
Brevoort talked about the fallout from the decision. He said that he still hears about the fight to this day, with some fans suggesting Marvel sold out with some conspiracy theories suggesting Busiek was angling for a DC Comics job and decided to make Superman win to give him better standing at the publisher. Brevoort said the fight and ensuing commentary surrounding it was a situation that fans clamored for, but no matter the result, they'd be unhappy. In the blog post, he says he didn't apologize for the decision since it was the "right outcome for the characters."
Ultimately, Brevoort is bang-on about his assessment of fan's reactions to the fight. No matter who won, there were always going to be people who were upset. It's all hypothetical until an actual outcome was decided on. Kudos to both Marvel and DC for making a unanimous decision that ruffled some feathers of diehard fans. While JLA/Avengers is a bit of a bloated story, you can't criticize for playing it safe. Superman and Thor fighting was incredibly cool - picking a victor was bold and turned out to be the right decision.
