I love the Mo Willem's Early Reader series about Elephant and Pig. They are sweet stories about friendship. Elephant and Pig are best friends though sometimes problems do arise like a broken toy or a misunderstanding. This started me thinking about other best friends in picture books and there have been some great ones!
The Frog and Toad series by Arnold Lobel is a classic. Each book about Frog and Toad contains several short tales about their adventures.