I am gonna have to go with the last hour of Crysis. I think the actual level is called Reckoning. Nomad flys the VTOL to the carrier in the midst of an epic downpour at dusk. You do your business on the ship and watch them prepare a nuclear strike against the alien base on the island against the will of Rosenthal, Prophet and others. Prophet says fuck it and peace outs. The strike launches, the base grows exponentially bigger in a huge explosion and the ship is under attack in the middle of the ocean, full siege. You finally make it back up to the desk to horrified screams of "HELP US! FUCKING HELP US!!!!" as you look up to see droves of aliens swarming the desk from all angles in what I consider to be the most epic FPS moment ever.