Hop on Pop. I've read it to my almost-two-year-old dozens of times lately.
I love how it feels so 1960s, in a way I never recognized as a child.
I also like the cozy feelings it gives me. I think this is the book that taught me to read. I memorized it when I was 2 or 3. I always insisted that my grandpa (whom I call Papa) read it to me, because of the title.
"Long Song / A long long song. Good-by thing you sing too long. "
That's always been my favorite part.