I guess this is a tough one to answer with many different aspects (only chain hotels? etc.)
as a luxury hotel freak I wonder which city will make me happiest - which city would be worth a trip only to wander around to see these fantastic hotels (not talking about Vegas style). Boutique hotels, great chain hotels etc.

My immediate (outstanding) guesses would be:
Europe: Paris (MO, upcoming Peninsula etc. etc.) & Rome for the Luxury Boutique hotels
Asia: Hong Kong, Tokyo
America: New York
I think there is no outstanding city for Africa & Australia.

with the overall winner being Paris.

What do you think?