I have to say A Friend For Dragon is super cute and the "If You Give a Mouse a Cookie" books are a must have along with the Mercer Mayer collection. I remember loving Big Clifford books when I first learned to read and acquired many for my daughters but I couldn't tell you why I liked them so much as a kid. Maybe it was the idea of having a giant dog all to one's self.

As an adult; i still love "The Velveteen Rabbit"  I enjoyed it as kid as well.  Something special and magical about it. 

I also loved "Where the sidewalk ends" and "Where the wild things are"...limiting to one is I think impossible!

The Secret Garden by Frances Hodgson Burnett. My Fourth grade teacher began reading in class, a chapter a day, and I couldn't wait...I made my mother take me to the library so I could check out my own copy. My mother was from Yorkshire so she did the "accent" parts for me so that I could learn how the language sounded to Mary Lennox. I still love that book and I'm now 64. I introduced it to my daughter first thing and she loves it too.

For a young child, I would say Chicka, Chicka, Boom, Boom!  Along with that, the Eric Carle books are classic (especially The Hungry Caterpillar).  My favorite children's authors are Dr. Suess (well . . . you know what I mean!), Eric Carle, Anna Dewdney, Karma Wilson, Sandra Boynton, Bruce & Audrey Wood, Laura Numeroff, Amy Hest, etc.  I loved the Berenstain Bears as a child & my son enjoys them too along with the Curious George & Thomas the Train books.  I'm sure the list could go on.  I love good children's books & there are SO many of them.

I have the one my aunt gave me in 1952, it is well loved, as are the ones I bought for my children, but now that I'm having grandchildren, I'd like to buy them each a copy.  Unfortunately, it's been out of print for many years, and costs hundreds of dollars on e-Bay.  Best stories, best poems, best rhythms.