All of the below is IMHO, and I'm sure everyone here will have their own thoughts...
I'd probably start with one of his Discworld books. Although they feature recurring characters and in-jokes, this doesn't mean you have to start with the very first. His style of writing and humour changed slightly over the years, meaning that some of the later books are much funnier than the very earliest ones.
Two "best book to read first" suggestions would be "Guards, Guards!" (introduces the hard-bitten, cynical misanthrope night watchman Sam Vimes) or "Witches Abroad" (which (ahaha) introduces the witches Granny Weatherwax, Nanny Ogg and Magrat Garlick).
My favourite of all time is the non-Discworld "Good Omens", which as TFA mentions was a collaboration with Neil Gaiman. An angel and a demon collaborate to prevent the Apocalypse, involving the M25, black nuns, paintballing, a Satanic Hellhound who likes to chase cats, and various "Americans and other aliens".
RIP Sir Pterry. We met only once, at a book-signing in Chester, and my pimply 20-something self was so over-awed to meet one of my literary heroes that I stuttered incoherently and made a fool of myself.