Picture books...
Anything by Sarah Stewart. The Library, the Gardener, etc. They're beautifully illustrated, typically feature strong/eccentric protagonists, and have surprising depth and often historical context for picture books.
All the Places to Love by Patricia MacLachlan. I cry almost every time I read this book. I don't even know why, just the sweet simplicity, the love evident in the family portrayed, the beautiful illustrations and narrative.
Twilight Comes Twice and Hello Harvest Moon by Ralph Fletcher. Beautiful, poetic, complex but graspable for young children.
ALLLLLL of David Wiesner's books. Tuesday, Flotsam, June 29, Hurricane (just got this one and I read it, I'm saving it for the coming months known as "hurricane season" around here!).
ALLLLLLLLL of Barbara Lehman's books; Rainstorm is a perennial favorite amongst my children but we love the others too.
The Frog and Toad books.
Robert Munsch's The Paper Bag Princess. Having a bunch of girls, I love stories that empower girls (even under the title "princess") rather than putting them in patriarchal boxes. I love most of Munsch's books. Not all that fond of "Love you forever," and some of them border on crude, but I still like most of them.
The Little Bear books (and TV show too... I usually despise preschooler TV but Little Bear has a special place in my heart).
Pippi Longstocking and the Noisy Village series... yes there are things we don't care for in how adults treat some children, but those references are so minor and don't impact the overall themes and fun of the books, so we love them!
All of Jan Brett's books.
Don't even let me start on independent readers and tween/teen series!