I second the vote for Make Way for Ducklings and Blueberries for Sal, (as well as One Morning in Maine, by the same author).
Others I can think of (without looking at my mom's book shelf):
Grandfather Twilight
Miss Rumphius (I used part of this book when giving the "Student Response" speech at my University's commencement ceremony last spring)
Flossie and the Fox
Stellaluna, Pinduli, and Verdi (all by the same author, whose name I can't recall)
Peter in Blueberry Land and others by that author (I think it is Elsa Beskow)
The Frog and Toad books, and the Owl Book by the same author (I think it is called Owl at Home)
And, even though these were mentioned in the "Awful Books" thread:
for little kids I think the Eric Carle books, The Very Hungry Caterpillar, etc are great. Also, The Runaway Bunny.
And we loved the "Frances" books, Bread and Jam for.., A Baby Sister for.., A Bargain for.., In fact, these were such a favorite for my family that when my adult siblings and I are at home and something isn't how it used to be/we expect it to be we joke that "things aren't very good around here anymore, no clothes to wear, no raisins for the oatmeal, I think I'll run away" (from A Baby Sister for Frances).
I am sure there I will think of more, so I maybe back...