I could never choose a favourite - I read SO MUCH. Most of the authors and books mentioned here, tbh.
Random other books/authors I liked:
Gene Kemp, especially The Turbulent Term of Tyke Tyler
The Trixie Belden books, especially the early ones
The Mandy books, by Lorna Hill
The Sue Barton books, especially the New York one
Trebizon and the Jill Robinson books by Anne Digby
The Kevin and Sadie books, and also the Maggie books by Joan Lingard
Flambards and the Pennington books by KM Peyton
The Cricket in Times Square by George Selden
The All of a Kind Family by Sydney Taylor
The Tillerman books by Cynthia Voigt
And many, many more.