I have to Go with what most say as well being The Covenant, The factors contributing to this being the following:-
Intro theme...those piano keys as the initial battle plan is laid out by Miranda Keyes
The Hornet Dog Fight section.
The Arrival of the Flood
Once the quote - "Brutes are mobilizing everything they've got"...and that Piano key hits again...MAGNIFICENT!!!
Get to the citadel, Up the lift...FLOOD truce (Gravemind makes Sense; I thought).
Arbiter getting Revenge and then the RIng rising from the Ark's forge....and that chant...need I say more.