http://www.youtube.com/watch?v=eYqnZuwk3q0&feature=plcp The entire Halo series in many regards is a build up to this point. Whilst there have been other antagonists during the plot, the Covenant are the number one enemy up until this point, where the killing of the High Prophet Truth, ends the Age of Reclamation, and arguably the age of the Covenant itself.
3. The Best Plot Twist of All Time (Halo CE)
You'd be hard pushed to think of another moment in gaming, and overall cultural history that opposes this scene. It's the climatic moment when you realise that Halo is about to become a fireball of 'Every man for himself'. Grab your AR, start running.