I've got one. Fallout 3.
::Spoiler warning::
I had started the purifier, providing clean water to the wasteland. I had freed prisoners on board an alien spaceship, and took the ship as my own. I had wandered Lookout Point and unlocked its secrets, taking out a maniacal brain that had been manipulating helpless people since the war. I had completed the virtual reality simulator for Operation: Anchorage, and I survived the Pitt, helping to push forward an immunity to all radiation. Now, I had just defeated the Enclave and destroyed their mobile base, leaving only roaming bands of units unaware that their home base had been obliterated from their own weapon. I had done it all. All that was left was to wander the wastes.
I didn't intend on playing much longer. I was the most powerful entity in the wastes. I figured I'd just wander a bit, see any areas I had missed, maybe roll over some raiders or save a few slaves. I walked on the outskirts of downtown DC at night, and ran across a four way intersection among some bridges. Mines lay in random places, old cars scattered around the area, and large group of high level Super Mutants roamed the area, just waiting for someone to walk by for them to kill. I began attacking them from up high, hoping to just whittle them down without getting hit, but they started to throw grenades, so I gave up the high ground and decide to engage a little closer.
That's when the Enclave showed up. They dropped a number of units wearing Hellfire and Telsa armor from their vertibird, which barely let the last trooper off before I blasted it out of the sky with my Tesla cannon. All of a sudden, your average scuffle with some Super Mutants just turned into an epic 3-way war. Missiles, grenades, and mini-nukes flew through the air, crossed by streams of lasers and plasma weapon fire. The cars each exploded in turn, taking out some of each group and crippling one or two of my limbs. The mines were hard to avoid as I dodged and weaved through the gunfire and explosions, using a combination of free fire and VATS when available.
In the end, it was me and a single Enclave solider, and one last plasma shot with my trusty A3-21 rifle ended the battle that probably could have been seen and heard from the Citadel itself.
There I stood, victorious, the night sky just starting to recede in favor of the approaching dawn. Fires burned from the wreckage of the ancient automobiles that had stood there since the war, as the bodies of my enemies lay strewn in the street among piles of ash and goo. I was at less than a third of my health, even after using a few stimpacks in battle.
I had hundreds of battles up until that point. Killed thousands of people, monsters, and creatures. I had single-handedly wiped out every last slaver at Paradise Falls. I had defended Big Town from the Super Mutants. I marched along side a giant robot, racing it to get to the enemy before it did. None of it compared to that single battle.
I wish I had saved it right before that point, or at least wrote down where that battle had taken place. It only lasted a minute, maybe two, but damn what a battle that was.