I have to go with Chief sans fireteam. Last I checked, when the darkness encroached his mote of dust in space, he kicked its arse back... basically single handed. Humanity made it much further into space on their own(-ish, with some forerunner meddling but that was degression) in the Halo universe.
No one ever mentions this but does anyone remember when the Librarian hooks him up with an evolutionary boost in H4. I would be curious to see Chief literally evolve into to something more, but this opens a whole new tangent from the question: What powers would Chief evolve to possess?
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What was that boost?
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People are still speculating to what extent he has been altered but it was at least enough to make him Composer resistant.