My favourites were and are the Swallows and Amazons series by Arthur Ransome. Odd, as I have no interest in sailing and have never properly visited the Lake District, but I love them. There are 12 books and they're all great. If I had to pick one, it might be Winter Holiday, or it might be Pigeon Post.
Loved all the Noel Streatfield books. Party Frock was a particular favourite.
I really enjoyed historical fiction as a child. There were several lovely books by Gillian Avery, probably out of print now. The Elephant War was a particular favourite. Cynthia Harnett is probably also forgotten now, but I adored her books set in England in the 1400s.
When Hitler Stole Pink Rabbit, by Judith Kerr (Meg and Mog, The Tiger Who Came to Tea). Read this when I was 11 or 12.
I was also very keen on Joan Aiken's book The Wolves of Willoughby Chase, and its sequels. I was entranced by the idea of historical fiction set in an era that didn't happen - now a whole genre, alternative history. I am Scottish and as a child was wholeheartedly pro Stuart and anti Hanoverians, so a world where the Hanoverians were the villains and the Stuarts remained our ruling monarchs appealed to me enormously.