I'd rather have one of the Baginses (not the Sackville mind you)
Normally you'd start with the first book from the Chrestomanci series, but I think The Magicians of Caprona (1980) and Witch Week (1982) might be the best for a new fan.
It's shame she passed away before writing much more and getting the fame she deserved :(
She had a unique and cozy style and way of world building and character design, where there is challenge and conflict but not deep malice or misery, if that makes sense..