Assuming that all forces of Halo fought all the forces of Star Wars, I'd have to say Star Wars. The size of the empire rivals that of the Covenant, and considering they have a cloning facility( eugh, prequel references *throws up*) they'd have an endless supply of Troops.
And, we have to take into account the biggest tide turner, the force. Assuming the ability still existed when the wormhole opened, the jedi and sith would be able to use the force on everything, meaning they can manipulate things.
This is a really basic summary, and could probably go into uber detail, but eh.