I will second the Bear Snores On series. I think we have 5 different ones now. Another one we read often is Hug Machine. And a seasonal one right now but read all year is Room on the Broom. We love Llamaphones as well. And Jabberwocky, from the little lit series that does many classic books used as themes for board books. One that is not technically a board book but introduced nath with very few words is Stack the Cats. And that reminds me of another favorite, None the Number.