The sight of hundreds upon hundreds of preserved corpses stitched together is unbelievably creepy. Not to mention the way this sight is revealed, and the accompanying music.
"That's not my scarecrow..."
See Eye Scream on the main page. In one version, you can even hear Darry screaming during the establishing shots before the reveal.
The scene where the siblings witness the Creeper throwing the bodies down the pipe. At first he doesn't seem to notice them as he carries on with his business, but then he turns around and just stares at them as they drive past. Trish and Darry very quickly decide to make a swift exit... and then they notice, to their horror, that the Creeper has started up his truck...
The idea of what the Creeper does to its victims. It takes them to a building that it calls the "House of Pain" and then rips them apart while they're alive and screaming, eating only certain body parts before leaving the victim to die on their own. Whether it needs fresh body parts or just likes to cause suffering is questionable, but it is certainly sadistic.