Firstly Verna Aardema, the author responsible for that magnificent pourquoi story Why Mosquitoes Buzz in People's Ears. This retelling of a traditional West African folktale won the Caldecott Medal for illustrators Leo and Diane Dillon in 1976. The stories by Verna that have stayed in print and demanded rereading are those that are based on African folktales.
For me, Dutch-born American Peter Spier has a standout book as well, the illustrations in Phyllis Krasilovsky's The Cow that Fell in the Canal. I love the humour of this book and Henrika's quest to see and be more than she is. It is out of print but still easy to find second hand and in libraries. Of course he has illustrated many other books and the ones that he has authored as well, such as Noah's Ark, Rain and Circus are special because they tell wonderful stories with very minimal text. Noak's Ark won the 1978 Caldecott Medal.
Thirdly, is Cynthia Rylant who is also an American author. In the library there is a book called The Whales which Cynthia both wrote and illustrated, but I haven't seen any other books that are illustrated by her. I think of her more as a author and a wonderful one at that as my first introduction to her writing was her Newbery Medal winning novel, Missing May. This week I purchased Snow which is illustrated by Lauren Stringer and it too has beautifully poetic writing.