Really difficult one, but I chose Alborada del Gracioso. I've always had the impression that Ravel's true individuality comes across a lot more in the more rhythmical compositions- not necessarily upbeat/fast although things like Alborada, Scarbo and the finale of Daphnis et Chloe are all fantastic compositions, just the ones that make interesting use of rhythm- the opening figuration of Ondine, the bells in Le Gibet and La Vallee des cloches, La Valse, the various other dances within Daphnis.
As much as the first three pieces of Miroirs have elements of what I've been describing above, they are to me not particularly far removed from Debussy. They are still great pieces and I probably listen to them as much as I listen to the final two pieces, but when I think Ravel I think rhythm.