Nothing is guaranteed to flower at Christmas. Witch hazel/Hammelis, Christmas Box, some Viburnams, Mahonia, Pansies and Primula come to mind. Depending on weather then Winter Aconite can be flowering at Christmas but normally Jan/Feb/March.

Hellebore........The christmas rose and indoor forced hyacinths if you start early enough

It isn't easy to get a Christmas rose to flower that early though, in spite of the name. Would probably need to be under a cloche?

It all depends what sort of winter we get........

Also you didn't say whether you meant an indoor or an outdoor plant. You can get Christmas cacti to flower at Christmas if you time the watering right but you might need several to be sure. Mine were all a bit early last year.