Comes down to Hive and Cabal because without timegates, the Vex cannot go back and forth through time and will eventually be wiped out. The Fallen would also put up a good fight, but they will ultimately be overrun by the Hive. The Cabal could win this battle by blowing everything up, but they'd have to deal with the Hive Gods in their Netherrealms. Unless... Everything from each faction is thrown into this battle, then the Cabal would just win because they would just blow up everything, including the planet that this war takes place on.