Also Black Beauty because it's just so simple and so complete a story. Rich, poor, kindness, cruelty all of life is there.
Mrs Frisby and the Rats of Nihm, because it is simply the only other children's book I ever bothered finishing.
(Except twins at st Clair's and they don't count.)
Again it is, superficially a very easy read, but like Black Beauty, it encompasses a whole life time of events and choices. It even has an ecological twist for today's youngsters.
If you have never read it Do!