Depends on what you value most!
For the most stable snowpack and comfortable temperatures, plan your trip for spring (March through May). This is the most popular time for backcountry skiing in the Alps. The days get longer, the weather warms up, and the base layer of snow becomes more consolidated, reducing avalanche risk. However, there can still be occasional cold snaps with fresh snowfall.
Want less crowds and potentially better powder? Come in the late winter (February and early March). You might encounter more variable conditions with colder temperatures and the possibility of fresh snow, but also potentially lighter crowds and untouched powder snow. Be aware that avalanche risk can be higher during this time.
In December, While some areas, especially glaciers, might have snow cover, conditions can be unpredictable with high avalanche risk and limited terrain options. Not generally recommended for backcountry adventures.