Two Betrayals for me. Would have been Assault on the Control Room, but that opening cinematic for Two Betrayals is amazing. Not only do you learn Halos true purpose, but now, not only do you have to fight the Covenant and the Flood, but Guilty Spark and his sentinels. I remember being 13 yrs old, and just having my mind blown that a video game could produce this much story, and still keep you guessing.