Presumably, you are are giving them a pleasant greeting to make them feel good? therefore:

If you think they celebrate Christmas: Merry Christmas is perfect.

If you think they are not of Christian background (Hindu, Buddhist, Wiccan, or etc.) then Happy Holidays may be more pleasant to hear.

If you are in a more formal, ~business relationship, the more neutral Happy Holidays is sure to be appreciated by all.

If they are a Scrooge, they will reply Bah Humbug! to either