The dispute over whether niqab is fard or not has been going on among the scholars since the earliest centuries of Islam, and will probably still be going on at the end of time. Nobody can claim to have a definitive answer to this question.
All sisters should research both sides of the argument for themselves and decide which side has the stronger dalils. Here are the dalils. Just so you know, I personally feel that niqab is mustahabb and sunna but not fard.