"Fat Slut". Pip's odd way of singing the lyrics, the lyrics themselves (which is essentially Pip's comeback to the titular insult), the odd guitar feedback, and the brevity of the track makes it awesomely ridiculous.
Also, the 'Brain Fart" improv she did in her 2007 tour.
Many of her improvs could count, as could some of her interviews. She's a funny woman.
Especially the "Tori Is a Lioness" interview. She claims that she would rather burn a man (especially if he's British) than a piano if she was stuck on a deserted island!