The simple answer is 'nowhere'. Snow can't be guaranteed in Scotland at any time (with the obvious exception of the indoor ski centre at Braehead!).
However the areas around the main ski resorts (e.g. the Cairngorms) are likely to have snow around Christmas. See the links here to view the current conditions:
http://ski.visitscotland.com/conditions/
Maybe "guaranteed "was the wrong word to use,should have been "usually".
I am thinking about taking the whole family to some snow next Xmas,,just seen a lettings in Aviemore,which appears to be a snowy area.
I know it is rather early but I love to plan early ...and ..it is just an idea formulating in my mind.
Aviemore is the main ski area in Scotland but the main ski (snow ) season is after Christmas. To get Guaranteed snow you need to go to one of the high altitude European ski resorts. (not cheap) or USA. Even in Europe the ski season is January to April.