Dr. Doom vs Darkseid
Both characters are from their current mainstream worlds. They have full use of all their feats and the fight is being taken place on a neutral battle ground, although both parties have had their complete lair's transported on to it. They both have a week to prep with full knowledge of the other.
Let the party begin...
http://images3.wikia.nocookie.net/__cb20111006002338/marveldatabase/images/0/04/Dr_doom.jpg
http://www.reviewersunknown.com/wp-content/uploads/2011/12/Darkseid.jpg