Powderhounds; in search of the best powder ski holidays
is a good site for this info
they say best powder resorts are:
Rusustsu Japan, Solitude Utah, Silverton Colarado,
they also list:
Hakkoda Japan, Kiroro Japan, Wolf Creek Colorado & Grand Targhee in Wyoming
As a Canadian I will also mention British Columbia interior Canada also good for powder , revelstoke, kicking horse etc.
This is for resorts. I think that the answer changes if you are talking about expedition trips or heli/cat skiing.
good luck with it, sounds like a great trip