I think Whistler is a pretty decent bet.
Banff can be iffy. We had a great season 5 years ago when almost everything (including the dive) was open in November. Snow was decent last season, but Avy danger was a real issue in the first half of the season. The other seasons have been hit or miss. In my opinion, the best month for skiing Banff is March..followed by April then February.
Fernie can be a decent choice for Mid-Late December. The BC resorts generally ski better than Banff from Mid December to Mid February/Early March.