David Walliams has said that his favourite book he's written is
"The Boy in the Dress". He has mentioned this on multiple occasions, including in interviews and on social media. He has described it as the book he's "most proud of" and the one that is "closest to his heart".
He has explained that this is partly because it was his first book and partly because it tackles important themes like gender identity and acceptance in a way that is accessible to children.