Peter Hartlaub, a contributor to SFGate.com's parent blog "The Poop" (what a great name!) posts that he never gets tired of reading Lucy Cousins' Hooray For Fish! to his four-year-old son, but may turn to violence if he has to read Goodnight Moon one more time. He then asks other parents to talk about the picture books they'll never get sick of. There's a great list of books here, from those who are most often exhorted to "read it again!".