The problem here is that he's written so many! How to narrow it down?
I love The Wee Free Men, purely for the Nac Mac Feegles.
I adore all the witch books, particularly Maskerade (well it is based on Phantom!) and Greebo is a total star in this one! But I also really like Wyrd Sisters.
However, the one I still go back to and laugh the most at is Mort.
The hapless Mort is taken on as Death's apprentice and predictably chaos ensues. But the whole concept of Death having a mid-life (?!) crisis, giving up the scythe and wanting a normal life is what makes it so funny, that and him having a horse called Binky!