Superman at full power and let's say that he is brainwashed by Darkseid. The X-Men consist of Cyclops, Wolverine, Storm, Colossus, Shadowcat, Lockheed, Nightcrawler, Beast, Jean Grey, Rogue, Gambit, Psylocke, Jubilee, Bishop, Iceman, Angel, Cannonball, Husk, Northstar, Polaris, and Havok.
They're fighting in New York City on live television since the Avengers are in outer space doing something, the FF are in the Negative Zone, Spider Man, Black Panther, Daredevil got their collective buts kicked, Elektra is in Japan, and Dr. Strange is fighting Mephisto. Everyone else is somewhere else.
Who do you think would win? Even if the X-Men got last minute help from the New X-Men, namely Pixie.