My favourites are books I also read as a child - a lot have been mentioned, but from Enid Blyton, The Five Find Outers. I still love reading those.
Just William is even better as an adult than a child; the language, the comic timing and how lovable William is (as a child you don't see that).
Billy Bunter - yes, problematic as utterly unacceptable in many parts, racist for sure and the fat shaming (I mean it REALLY was fat-shaming) ... but so well-written and the characters were hilariously drawn. I read them all as a child but from the Library. We bought only a few. My DM and her siblings had loads, which we would read in my DGM's house; they all got taken by one uncle without a word, and I still wish I could get even one or two. I found a couple in Edinburgh in a second hand book shop some years ago, when I was on a visit. I was thrilled!
From a very small child perspective, one of my favourite children's books is 'Library Lion', Michelle Knudson. Read many many times, and I still take it out and read it.
The others that I love still, couldn't claim to 'read' them, are any of Emily Gravett's books, Monkey & Me a particular favourite. My eldest loved them so I can't quite pick them up without being transported back in time to those early days of reading to her. Long time ago now!
And finally, The Little Princess, Frances Hodgson Burnett. I love it. Always have.