User blog comment:James12708/Reapers VS. Forerunners VS. Angels from Darksiders VS. Humanity from Halo VS. Council and humanity from Mass Effect VS. Imperium of Man/@comment-5267262-20130116194644

Largely this actually boils down to who does the best scorched earth war, in which case I hand it to the forerunners since the halo rings are (supposedly) able to wipe the galaxy clear of life.