Matilda for me, although James and the Giant Peach is wonderful. Reading his stuff as a kid is something I really miss. I remember feeling dazzled by the imagination and the cruelty and the humour. His writing taught me the importance of joy and sarcasm. So good.
His grown up stuff is pretty good too. Cruelty in particular is a superb collection.
There was a really, really low budget creepy animated version of The Magic Finger that I had on VHS as a kid which I’ve tried for a few years to track down now, to no avail. Still freaks me out a bit now thinking about it.
Feels difficult to nail down which one is his best. I remember my entire childhood being consumed by these really dark, sinister stories and was absolutely obsessed by them.