From Origins: Part 1, the Forerunners going to war.
The battle in "The Duel". One Arbiter vs. Hundreds of Covenant, including a wraith and 3 supersized Hunters? Curb-Stomp Battle on the troops, 3-0 against the Hunters, and I'm pretty sure he took out one by throwing the Wraith at it...
Prototype. A marine uses a prototype, upscaled Mjolnir suit with built in weapons, and uses the time left before it's self-destruct mechanism activates to allow his men to escape. The suit resembles a Gundam, in both appearance and the improbable amount of Covenent it takes out. Bad Ass on so many levels.
To be precise, it's actually based on one of the Marauder Suits from Starship Troopers, both in design and combat style.
"Darkness born in eons past became a threat to all life once again. And humankind and the Covenant who had drowned for so long in each others blood, were united if only for the briefest moment against a terrible enemy they both shared. Righteous souls continued to fight, believing that they could defeat the darkness. And as they fought together, they stumbled together, into peace." All the while, we're shown scenes of the Covenant and the UNSC fighting alongside each other against the Flood, while Sacred Icon Suite 2 blares in the background. Badass.
The sequence in Origins of the Covenant and Humans fighting against the Flood. Epic.