Or Philip Pullman
Though as a lad I was reading Hammond Innes, Alistair MacLean, Ian Fleming, JRRT, HG Wells amongst others, never did me no 'arm, 'onest guv
TBH I'm not a fan of "childrens authors" for "young adults" as long as there is no explicit stuff, and not overly sweary it's probably not much different from school-break talk these days.
On JKR - yeah, OK, reasonable storyline, but bloated like late-on Marlon Brando in the Island of Doctor Moreau (which incidentally I was also reading as a ten year old)
George RR Martin - I picked up the first GOT novel, read three pages and put it back. I very rarely don't finish a book I start (Vikram Seth's A Suitable boy being another example, I got halfway through that)