Also stolen from that article (gawd that series was great, 343 should totally hire those guys), The Ark has one of my favorite moments in the series. After a brief but joyful romp in the Scorpion (tank beats everything), we run into a narrow pass and have to hop out on foot. This is typically where Halo missions choke off the tank progress, and we move onto something different. But in a wonderful head-fake, we're just getting started, and open the gates to see this massive battlefield, with tiers of enemy ranks lined up, and a Scarab stomping right overhead. And we're presented with several ways to advance. The Gauss Hog? The Scorpion? Try and snag a Chopper? Head to the central structure and rock the FRG?
And the the Scarab stomps into the picture again, climing up over the citadel on the horizon and marching back into the battle.
I love the anticipation of seeing it climb back over into view, and slowly march into the fray. The animation as it climbs down from the citadel is so great.
The ongoing battle, the absurdly gorgeous skybox, with the Ark stretching out in all directions, the massive horizon-spanning waterfall off the cliff shelf....the Halo 3 mission desginers were on some potent stuff.