My daughter and I were in a Borders and found a book called 1001 Books to Read Before You Die. It was arranged chronologically by millennium and then century. I had read a decent number of them and my daughter had read a few. When my wife finally dragged us out of there, my daughter asked if Laura and I would put together a description of at least 100 books she should read and why. Two I know I will include for very different reasons are "The Great Gatsby" and Asimov's "Foundation".
Obviously we have very different kids at very different stages of life but if your kids asked you the same question, what would you include?