This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Good Omens
What Hastur does to a room full of hapless telemarketers when they inadvertently release him from the electronic prison of Crowley's ansaphone. He basically oozes through all the little holes in the earpiece as a stream of maggots, which assemble themselves into a single huge, angry maggot that starts to feast upon the telemarketers in the most disgustingly horrific way. He basically eats them alive, burrowing into their internal organs and stripping the flesh from their bones in a heaving, writhing mass of screaming, dying flesh and maggots. You could almost feel sorry for them if they weren't telemarketers.
Crowley finding out that Hell knows he's betrayed them.
"Crowley...we will win this war, but even if we lose, at least as far as you are concerned, it will make no difference at all. For as long as there is one demon left in Hell, Crowley, you will wish you had been created mortal. Mortals can hope for death, or for redemption. You can hope for nothing. All you can hope for is the mercy of Hell." .... Crowley turned off the radio and bit his lower lip. Beneath the ash and soot that flaked his face, he looked very tired, and very pale, and very scared.
The scope of Adam's power. Sure, he's mostly benevolent, but he was unconsciously controlling his entire hometown for years.