Three come to mind, all in Left 4 Dead.
3. Versus mode. Both teams full, both mic'd, both have their shit together. Best way to play any game, right there.
My brother, brothers-in-law and I are Infected. Survivors nearly get into a safe room. One guy gets pounced twenty feet from the door. His teammates turn around to rescue him, setting off a battle of inches that lasts a solid ten minutes. We drag a guy to the car, setting off the alarm. They deal with the horde, get everyone standing, then it starts over again. Back and forth, back and forth. Twice all but one of them actually get inside and shut the door, only to open it to rescue the incapped guy. On and on it goes. We kill one Survivor, then another, then a third. The last Survivor runs into the safe room alone -- only to be pounced from inside the safe room by my brother, who'd crept in there unseen a minute earlier.
The other team wasn't even mad. We all complimented each other.
2. Blood Harvest, Versus mode. I am Survivor.
We have a bad time of it during the finale. Tank anticipates our molly, other Specials take out two of us in hard-to-reach positions. Suddenly it's just me -- and I survive, by myself, to kill the Tank, three Boomers, three Smokers, four Hunters, and god knows how many Commons. For five golden minutes I am invincible. I do eventually die but five players compliment me on how well I did.
I imagine that's what champion players feel like -- Zen awareness, spinning and shooting just there because you know without seeing that there is a Special there. You already know what he's going to do and you already made him walk into your bullet.
1. Dead Air, Expert.
Again with brother and brothers-in-law. We try the finale a dozen or so times, each time having an "aw, no, no, NO" moment ending in complete crash-and-burn. Nobody survives to get in the plane.
We finally have a minute of brilliant synergy, get through the second Tank with everyone alive, and run for the plane. Me and two others go up the ramp, but one brother-in-law is backpedaling to shoot at the horde, not looking where he is going. We turn and watch in horror as he misses the ramp. He is immediately surrounded by Commons and incapped.
Now, what we should do here is write him off and finish with just the three of us. "Sucks to be you, Kris" we should say, as we lean back in our chairs, lighting up cigarettes.
What we do instead is charge, all three of us, out of the plane to rescue him. And we do it. Surrounded, badly hurt, Specials closing in all around, we fire off the last of our ammo clearing the immediate area. We help him to his feet, then he's down again. A Smoker grabs me. Someone headshots it. By some miracle we get our buddy standing again. We shoot a Hunter out of the air as it pounces.
All of us limping, two of us black-and-white, three of us out of ammo, Commons all around, another Tank almost in striking range, all four of us limp onto the ramp, shoulder to shoulder, like a scene out of a war movie. Roll credits.
We got up out of our chairs and cheered. First Expert campaign any of us ever finished.