Every season is different, however you can have great conditions both at the beginning and end of the season.
Temperatures and the amount of daylight tend to be better later in the season, but better quality snow may be found at the beginning and in the middle of winter.
Choosing mid-season, from the end of January through to the middle of March, should guarantee you the best of both worlds. Some lower resorts can suffer a lack of snow at village level in very early or late season. Choosing a glacier resort like Les Deux Alpes will ensure you never have this problem, with skiing for all levels of skier available at higher levels virtually all year round.