BFME 2, the greatest RTS of all time. I was playing as Mordor, and doing this neat little thing I always do where I choose my base to be inside of Helm's Deep, which gives 2 layers of massive walls for protection. I choose to have 1 ally, the Elves, and go up against 2 allied factions, both of them are the Dwarves, the most OP faction in the game.
I had the Witch King as a hero unit, and he is the most OP hero in the game. I love watching him level up and continuously counter everything the enemy throws at him. Got a lot of units in your army? He deals massive area damage in his regular moves, instantly killing regular units. Powerful melee units and siege weaponry to crush him? He can ride on a fellbeast, out of reach. Ranged units? He can use Screech, making them flee in terror. Buffs and upgrades to your units? He passively halves the damage and armor of all units in a large range around him. Other heroes to counter him? The "Hour of the Witch King" ability sets all special abilities to just used, forcing them to recharge before being usable. It was awesome.
Eventually, I got enough power to summon the Balrog, so I sent the 3 musketeers: Balrog, Witch King, and Sauron, all to the enemy fortress, and they not only destroyed it, but we're in good enough condition to take down the second fortress on their own as well. Dwarves ran around helplessly as their buildings crumbled under my heroes' power. At the last moment, with one Dwarven building left before I destroyed it and won, I had all of my units attack it at once. The Balrog unleashed his fire breath ability, which unfortunately is friendly fire, and killed the Witch King just as we won, which meant that I had the Lord of the Nazgul exploding as my victory screen. I had no expectations of that happening, but I was satisfied with that, because it was the best battle I ever had.
Too bad the Elves died instantly.