COD4: - The Nuke going off was amazing, and walking through the destroyed city after crawling out of the helicopter was chilling.
- When you shoot down the chopper in One Shot One Kill and it crashes into the building and comes towards you propeller spinning. That was awesome.
Gears of War: - When you're on the train in the last stage, if you look up at the sky, it looks freaking mezmerizing.
Army of Two: - Same as the Gears of War reference, but it's the water when you're on the aircraft carrier that looks awesome instead.
Other than that, the most beautiful sites for me are when you're either escaping certain death by the skin of your teeth (One Shot One Kill's helicopter rescue for example), or when you're rushing into a huge battle that you know is gonna kick ass, like Zone of Enders 2's Airship Battle.
OH, or when you're working with a team, be it actual players or A.I., and you're all just plowing forward towards your enemies or objectives. It's a beautiful sight. I love those first seconds of a multiplayer match of C.O.D. 4 when everyone loads up and then rushes out, or if you play a mission in Armored Core For Answer where you have a teammate, and you both just hit the overboost at the same time and rush a huge Arms Fort. It's sweet.