Covenant easyily will win against all. They were kicking the butts of the UNSC in the novels (4th book might change that, but we have to see). According to the books for the Halo universe any of the larger battleships of the Covenant can destroy all life on the surface of a planet with their plasma weapons. But their Troops are not as cunning or smart as the humans of the empire or UNSC. They basically copy what they see and use it to the best of their knoledge. Also they have the ability to make pinpoint jumps across systems that are arguably better then the empires ability.

The UNSC could easily kick the butts of the Empire. Their ships are fitted for war against the covenant, and other advanced weaponry. They have a limited number of fighter ships, but i can bet carrier class cruisers would come into production prety quick. The only problem is that in the books and game thier ships and weapons work with a knoledge of the laws of physics. The Empire hs the ability to avoid that with their ships by allowing them to work like airplanes. So there is a disadvantage there. :-P

Then comes the A.I. systems of the UNSC ships. They are actually smart and if more3 of the Cortana level are created, can actualy improve on any captured technology they see.
And of course the UNSC spartanprograms. Storm troopers move over, here are the real super troops. :-)

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