The Precursor/Ancient Human/Forerunner period up to the Reintroduction period after IsoDidact fires the Halos, without a doubt. It covers 10 billion years of history, it could stand as a separate universe itself. The whole period is like a twisted, Lovecraftian cosmic game filled with personal tragedies, mystery and awe - moreso than the rest of the series. The only post-war plot thread I have really enjoyed is the small snippets we've seen of ONI gradually piecing together the Precursor/Ancient Human/Forerunner story through Bornstellar, Chakas and Catalog. The Insurrection and Human-Covenant war got dragged out for so long it got pretty dull.