II'll give it to you that in a battle between droids and Flood, the droids would win, But you don't get to pick and choose what elements of your universe are there, so the droids would be fighting alongside living creatures, giving the flood endless reinforcements. A single Halo ring could destroy an entire galaxy when set at correct power, a Sun crusher would have to take it one star at a time. A Galaxy Gun really doesn't apply against Promethean weapons, as it belongs in the superweapons category, where it is once again defeated by the Halo network. At first glance, I too thought it would easily be Star Wars, but when you account for all the variables, the Halo universe is just full of so much intense unexplainable power.