Mine would have to be the scene which starts of the level in Halo 3 "The Ark".
Nothing says "badass" more than having the Elite ships engage the Brute ships whilst the humans drop onto the Ark.
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Also the whole "Inside Job" cutscene from Halo 2 (Gravemind opening) is incredible - from the mysterious poetic machinations of Gravemind himself, to a glimpse at 2401, and best of all, the shot of High Charity over Delta Halo at the end, depicting the break-out of the Covenant Civil War, set to Marty's beautiful string piece from "Cairo Suite". The whole scene is a literary and cinematic masterpiece. I love the irony of S117's outright appearance as Truth declares that "nothing can stop us".