Halo 2's High Charity. That mission had so much chaos, and was just loads of fun. Having a bunch of species fighting each other while you are one of the only humans inside the Covenant Holy city is exhilarating. Not to mention being able to see all the fights happening between Flood, Brutes, Drones, Grunts, Jackles, and Hunters. It just defines "Fun Chaos"
Not to mention this vista to your left most of the time: