The Hundred Dresses, which is a beautiful story, but it is really the Moffats and the Pyes that put Estes toward the top of our favorite author list.
We discovered Ginger Pye a few years ago at a charity shop in Scotland. After reading and rereading it several times, we quickly found and read all of the Pye and Moffat books,and while Ginger Pye remains our favorite and is always the first one I lend to people, they are all very good and have formed a bit of family culture and imagination. Jim has mentioned how he, like the Pyes (in Pinky Pye), would like to rent a cottage on an island somewhere to write for a summer, and we all are able to picture that summer and heartily agree. And, all I have to do is mention the Middle Moffat's organ concert to get a grin from Jonah!
Estes' books are a slightly higher reading level (9+), but so sweet and simple, there is no one too young to enjoy them (as long as they hold his or her interest). Even though Estes wrote these books over sixty years ago, the simple humor and the oblation of everyday life of children and their families make these books timeless.
Note: These are great books to keep an eye out for in thrift shops. I've picked up a couple just in the past month (they went into our 'give away' stash of good-condition used books, which we use for gifts.)