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-4721065-20130112104250/@comment-1683193-20130112184042

The Halo's should take care of Orks and Tyranids fairly handily. Forerunners vs Necrons could be an intresting fight,though, as Necrons would be immune to the Halos and are the most advanced 40k race.