I was pretty surprised that I didn't see this one because it was such a popular game, even after scrolling through most of the answers here.
One of the earlier missions on Call of Duty: Modern Warfare 2 was called “No Russian” and it made sure to tell you that you could skip it with absolutely zero negative impact to the story/your trophies. But I just said what the heck, ratings and warnings never scared me before.
The mission starts with you on an elevator armed to the teeth in a bulletproof vest and holding an LMG (light machine gun), and several other little High quality elevator goodies (machine pistol, grenades, 800+ rounds of ammo). On the elevator with you are several other equally armed “brethren”. As the elevator dings into place, all you hear is one of your comrades say calmly, “Remember, no Russian.” Then the doors slide open and you're presented with a large line of people facing away from you in an airport. Civilians, waiting to get their bags checked and go through security, as well as a handful of TSA agents to do the securing. You and your posse stroll out of the elevator at the most lackadaisical pace, when slowly some of the people start turning around noticing this group of Rambo-esque men waltzing towards them. After a few seconds they start to realize what's happening and you see one security officer start reaching for his gun. That's when your machine gun suddenly pops up and you’re able to help your buddies mow them all down.
But wait, the fun doesn't stop there. This is a huge airport, with a lot of people, and you've got a lot more ammo. Time to go for a walk. The game keeps you from running so you're forced to walk at this ants pace, blasting any and everyone you see off their feet into oblivion.
Here is the video for those that dare, the mission starts at the 1:30 mark.