What were debussys most famous works?

Many opinions, so here's mine:
Claude-Achille Debussy was experimenting with a new tonality, the whole-note scale, where every step is a whole note (the major scale runs whole, whole, half, whole, whole, whole, half). It's a marvellous effect. Pieces that come quickly to mind are:

Prelude to the Afternoon of a Faun
Claire de Lune (Moonlight)
the Girl with the Flaxen Hair
la Mer (the Sea)
a string quartet

Two close contemporaries exploring similar kinds of sound were Maurice Ravel and Gabriel Fauré.