It depends on the time, and if they are working together or are fighting like normal. If it is present, and working together, then Star wars. they would just own the unsc and covies (yes, the flood have been eradicated as far as we know). If it is all time and everyone is working together, then forerunners, ancient humans, precursors, and flood would win, even against the Rakata and whatever. The precursors were intergalactic (civilization tier 0), and the rakata "would expand in thousands of light years in every direction"(wokeeipedia) (civilization tier 2). This isn't even counting halo rings and shield worlds. Plus, "one flood spore could destroy a species" they could just drop some infection forms on undefended worlds, then start working away on everywere else.
Currently, it would be star wars. The whole universe, it would be Halo. Fo' sho'.