Dae-su returns to the building where he had been imprisoned for fifteen years to find out who had done it and why. On his way out, after performing dental surgery on the guy-in-charge with a hammer, he beats the living daylights out of about twenty guys with said hammer, his fists and a knife stuck in his back. He then proceeds to beat the living daylights out of four or five more guys in an elevator - still with the knife in his back. Awesome...
For the director as well, given that the scene is nearly 5 minutes long, and is all filmed on one camera, in one take. The fight sequence took three days worth of attempts to nail.
There's also the scene in which the main character anticipates his enemy's trap and kills two men with a toothbrush.
"If you don't imagine, you won't be afraid of anything."