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Middle of January is usually the best time for consistent snowfall. Niseko is getting ridiculously busy these days. It's convenient for a base but it's expensive and a mini Australia. Rusutsu is a good option. It's starting to get busier, but nothing like the crowds of Niseko.
There are a number of other small ski hills in that area which are do-able as day trips and are usually much quieter.
Drop me a message if you want any specific advice. (About Hokkaido. I don't really know too much about Honshu.)