Do Catholics believe that Mary is above all other humans in holiness? Does her being born as she was (i.e. immaculately conceived), and being the Mother of the Savior (Mother of God) make her higher than everyone else? Or is she considered not necessarily holier than any other human being? Is there a standard Catholic teaching on this question?
Mary's dignity and all of her privileges follow from her vocation, which is from all eternity God chose her to be His Mother, therefore this places her in a realm distinct not only from all the saints, but from all the angels as well.


  The Virgin Mary, who at the message of the angel received the Word of
  God in her heart and in her body and gave Life to the world, is
  acknowledged and honored as being truly the Mother of God and Mother
  of the Redeemer. Redeemed by reason of the merits of her Son and
  united to Him by a close and indissoluble tie, she is endowed with the
  high office and dignity of being the Mother of the Son of God, by
  which account she is also the beloved daughter of the Father and the
  temple of the Holy Spirit. Because of this gift of sublime grace she
  far surpasses all creatures, both in heaven and on earth. At the same
  time, however, because she belongs to the offspring of Adam she is one
  with all those who are to be saved. She is "the mother of the members
  of Christ . . . having cooperated by charity that faithful might be
  born in the Church, who are members of that Head." Wherefore she is
  hailed as a pre-eminent and singular member of the Church, and as its
  type and excellent exemplar in faith and charity. The Catholic Church,
  taught by the Holy Spirit, honors her with filial affection and piety
  as a most beloved mother. - Dogmatic Constitution on the Church,
  Lumen gentium, 53


And she is holier than they because she placed no obsatcle to this unique grace from God, saying yes to God all they days of her life, faithfully corresponding with all graces:


  And Mary said, “Behold, I am the handmaid of the Lord; let it be to
  me according to your word.” - Lk 1:38 (RSVCE).


Many saints and doctors of the Church consider that the initial grace in Mary was greater than the final grace of all other beings.