I think any one of the defense missions in Warcraft 3. The first, where you hold out for reinforcements I kept building farmhouses, distracting the final impossible rush of enemies to the castle you had to protect until the mission ended.
The second (Under a Burning Sky I think) all of my defenses caved in. In case you haven't played it, you are summoning a kind of God of Chaos through a portal. As the portal grows (You have to defend it for 30 minutes or so) certain bonus units come through, all of which I saved at the portal. I also got these land mines, formed a perimeter around the terrace the portal was on, which took out some of the final rush of enemies. I also drew several groups of my units and the hero unit you have during the mission back to the portal, and for the last couple minutes of the mission, there was this massive war going on around the portal between humans, dwarves, demons, and lots of zombies (which you could summon from corpses). That was epic.
The last defense mission, I never finished without cheating, but when I went for a clean playthrough, the result was always epic. You were elves fighting along side human and orc forces, but your companions all die by the end of the mission and you are left alone against literally hundreds of the demons and zombies you had played as in the last mission I mentioned (you see, the elf story more or less has you fixing all the shit you caused in the undead missions).