The opening stages of the Battle of Earth in Halo 2 - first three levels, fighting the Covenant as part of an organised UNSC war machine in tightly-designed levels with awesome engagements. Seeing the Paris-class come down and rain death on the Covenant during Tip of the Spear in Reach is a close second.
Halo? Worst? These two words do not fit together :P
Pfft, dunno. I'll think of somethin'.