I say canon and after all of this 52/OYL/Civil War stuff is cleared up.
Forget the previous crossovers with Storm beating Wonder Woman and whatnot, and do it for real. And none of that "other circumstances" things where like Thor's hammer doesn't work in the DC world or something. Make it consistent with their comics, and don't cop out on fights (and certainly no fan votes).
Superman vs Thor = would be a good fight and in my opinion, Clark wins
Batman vs Captain America = Batman wins
GL vs Silver Surfer = depending on which GL (cause if they go with Ion, SS is screwed lol). This would be a great fight.
Grundy vs Hulk = Hulk would win (imagine the dialogue in this...haha)
etc etc etc
The trick for this to be awesome, though, wouldn't be just a series of fights - it would be an actual storyline that is longer than 5 issues. I'd like to see a merged universe, where nobody is quite sure how to get out of it, but they start making do with what they have. This leads to characters like Deadpool and Deathstroke trying to take hits on opposite universe people that they aren't used to, plus a lot of arguing over power struggles (for instance, no doubt Superman would want to be the leader of everyone, and so would Captain America) It would also lead to odd team ups, like Nightwing and Spider-Man patrolling through New York. Political conflicts dealing with issues like mutants being dealt with in the DC universe instead of Marvel, Iron Man's tech comparison to Batman's...so forth. It would also have to settle a lot of debates as to whether or not Mxy has been to the Marvel universe, what relationship Imperiex has to Galactus, who would want to rule over Atlantis (Namor or Aquaman), etc. Make it so that if the companies ever merged, this would set up a little backbone for it (though I doubt that'll happen and kind of hope it doesn't, since that'll just be a virtual monopoly and allow bad writers to come on board)