Whistler and Banff are always fun, though full of 18-25 yo Australians
Aspen, Vail and Breck have good apres scene. Aspen and Vail are a little more well-heeled, Breck is a younger crowd.
South Lake Tahoe is definitely fun. Being able to walk across the border to 24h boozers & casinos can be dangerous >

Salt Lake City has a surprisingly decent bar scene. It's not technically 'apres' but you drive back from the mountain, shower up and hit the bars
Jackson Hole's cowboy bars are a good laugh.
Overall though, North American resorts can't hold a candle to the likes of Ischgl/St Anton/Val D'Isere for the apres scene