Start by going north. Hokkaido gets cold before Honshu and keeps its snow driest, so for a December trip the island is the answer almost every time. Niseko and Rusutsu are open and snowy from early in the month, while the inland resorts like Furano and Kamui sit in some of the coldest air in the country, producing the lightest powder of the season. Honshu resorts can be excellent later, but early on they are a gamble next to Hokkaido's reliability.
Decide how much you value scene against solitude. Niseko gives you the full international resort, with night skiing, English everywhere and the easiest logistics, at the cost of crowds even early. Rusutsu, Furano and especially Kamui trade amenities for space and quieter slopes. If this is your first Japan trip, Niseko removes the friction, if you have skied here before, the quieter resorts reward you.
Mind the early season caveats. Cover can still be building low down in early December, tree runs need enough base to be safe, and some resorts manage off piste access strictly, so check the gate rules and never duck ropes. Always carry avalanche kit in the trees and read our avalanche safety basics for off piste. For the wider picture, see early season in the Alps and early season in Europe, and plan the trip from the Japan destination guide.