Perhaps Granny Weatherwax's finest moment: with a feedback effect—"I ain't been vampired: you've been Weatherwaxed."
Also doubles as Nanny Ogg's finest moment. She's the one who gave Esme the idea, with the comment about Weatherwaxes never being beaten - because it's "in the blood". Depending on your interpretation of Nanny Ogg's character, of course.
Mightily Oats deciding to make a holy symbol of his axe.
"An axe isn't a holy symbol, you stupid man." "Oh. Then let's make it one."
Another possibility is that he has realized that everything is holy, as long as you recognize it as such, and believe in it. In the same way the vampires realized that the holy symbols they'd taught themselves in order to guard against them could be seen in everything if they looked hard enough, because it was just a recognition of a pattern. It wasn't the symbol that made it powerful, it was the power of belief that symbol received.
Alternately, Om was watching, and thought that it would make a terrific holy symbol.
When Magrat makes Nanny Ogg shocked with an innuendo.