Not necessarily my favourite but it was possibly the one that got me hooked on Discworld way back near the beginning;
The scene where Bravd and Weasel are on a hill, discussing the fire in Ankh Morpork, was directly parodying the scene from one of my favourite author's books - Fritz Lieber's Swords series where the great city of Lankhmar was burning. He managed to lampoon enough of my favourite books and authors while twisting tropes and clichés without giving offence was enough for me to become a fan.