Mine would be Age of Ultron versus Doctor Doom.
What would have happened at AoU if Victor von Doom had his full faculties? Maybe he was already restored after the Fall of the Hulks mind wipe? My thinking is Doom had the Time Machine, and would have gone back in time to find how Ultron got so powerful, and if that fight didn’t beat Ultron, then Doom could have gone to another dimension or another time to fix things.
What’s your ultimate battle?