gulliver's travels - there's more to it as an adult and plenty to discuss - man's inhumanity, science gone mad, etc
north & south for something with very modern resonances - industrialisation, unions, public health...
the best 'classic' book we've read was william's a streetcar named desire, it's got everything - racism, class system, death, depression, madness, paedophilia, suicide, alcoholism, domestic violence, rape, mysogyny, and much much more - very compact, short (quick to read), simply written and incredibly powerful