Hey, great question! For me the first that comes to mind is definitely Philip Pullman’s His Dark Materials trilogy — amazing worldbuilding, compelling characters, awesome dark mysterious fantasy plot that stays strong across three novels — I reread all three recently and it held up really well.
I haven’t read any Diana Wynne Jones yet but will have to borrow a couple favorites of yours
Others that stand out…
Well, Harry Potter, but what millennial wouldn’t say that? Actually a series I started reading earlier and thus was probably more formative is Redwall. Rollicking good adventure epics with a cast of woodland creatures and famously sumptuous feasts! Haha I probably read around a dozen of those, I think I still have them somewhere…
Oh, also Charlie and the Chocolate Factory…and the underappreciated (or maybe justly less appreciated) sequel Charlie and the Great Glass Elevator. Lots of great Roald Dahl, his story collection The Wonderful Story of Henry Sugar and Six More I remember pretty fondly too.
Not exactly childhood but in high school (AP English summer reading project I think) I had to choose an author and read at least 3 of their books. I chose Gabriel García Márquez and really loved One Hundred Years of Solitude and Love in the Time of Cholera. I should probably re-read them at some point!