Three best parks in North America in no particular order are Woodward Copper, Mammoth and Whistler and no one comes close. Breck and Keystone used to be sick till Vail fucked them. Park City is dope and yes people will say that PC is on the same level but no way. The park run is really short which is good and bad; for hot lapping it's great but they just don't have a ton of features like the other spots plus they cram features real close. Some people prefer this but it's all preference. I would say PC and Aspen Snowmass are a step below the top 3 but they round out the top five. Winter Park should be in the top 10 and then from there is gets difficult.
I've been to most of the notable mountains in the U.S when I was living in my van so I would throw up Telluride, Bear mountain (but just for park), Brighton, Carinthia, Woodward Killington, Loon, and maybe a spot or two in Tahoe. Two spots I haven't been to that were mentioned are Mt Bachelor and Seven Springs. My opinion is based on the quality and the amount of features, how creative the park is, how often feature's get changed and switched around so the park stays fresh all season, and the different sizes of features such as medium, large and XL features. Would like to hear others opinions and some places I didn't mention.