Which one would you pick?
I'm expecting the majority of the answers to be Ghosts of Onyx.
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Halo: First Strike would make an awesome game since the story does a perfect job of connecting the events of Halo: Combat Evolved and Halo 2 together. On a slightly unrelated note, I could also see The Fall of Reach (that completely ignores the events of Halo: Reach) and Ghosts of Onyx having awesome movie adaptations.