Aya getting up and shooting Hyde at the last moment. Hearing her much more self-assured voice along with her Big Damn Heroes save made the whole game worth it for this Troper.
For most of the game, we've seen even the most elite soldiers on the planet slaughtered by the Twisted. In the final stage, what remains of the Coalition forces are scattered, leaderless troops, trapped in the enemy's base, just trying to go out fighting. They form up into organized units, work together and they kick ass.
In Operation Red Fog, you get on a stopped subway train which then starts moving and a bunch of Twisted come out. Then two Delta Force troops jump onto the moving train to give you some backup.