I can only think of best three with no clear winner. Assault on the control room, 343 guilty spark and the Maw.
Though pillar of autumn gets close, its the one that started it all.
343 guilty spark was just seeing covenant flee in terror, shotgunning some flood faces (i guess those could be counted as faces?), and fighting through flood hordes with marine survivors.
Assault on the control room because tank beats everything, not really that but because it had varying choices of vehicles, soloing around and fighting alongside of marines, it also had varying enemy classes from covenant ranks.
The Maw because of the different factions fighting with various enemy classes and how i could mention maw without The warthog run with capital "T".