Pillar of Autumn because it's the begining of the game.
No wait, Halo, because all the exciting things at it, seeing the ring for first time.
No wait, Truth an Reconciliation because the sniping and infiltrating in a enemy ship.
No wait, The Silent Cartographer because the Epic battles at the beach, a lot of hunters, and that silly Zealot behind the door.
No wait, Asssault in the Control Room because you get to drive a tank, a ghost, the snowy environments and so much mystery.
No wait, 343 Guilty Spark for the plot twist, the Flood and that little recording in the helmet, also the swamp.
No wait, the Library because it super action packed, close quarters and you get to see the sacred icon.
No wait, two betrayals, because the epic battles between Flood and Covenant, Banshee driving and fighting Sentinels for first time.
No wait, Keyes, because the darky environments, the way keyes was transformed into a proto gravemind.
No wait, the Maw because the Warthog run, epic ending for an epic game.
cwutididthar?