Sankara was the exponent of the Kevala Advaita philosophy, Ramanuja of the Visishtadvaita philosophy, Madhvacharya of the Dvaita philosophy, Vallabhacharya of the Suddhadvaita philosophy and Nimbarkacharya of the Dvaitadvaita philosophy. All were great souls. We cannot say that Sankara was greater than Ramanuja or Vallabha was greater than Nimbarka. All were Avatara Purushas. Each one incarnated on this earth to complete a definite mission, to preach and propagate a certain doctrine, which was necessary to help the growth of a certain type of people who flourished at a certain period, who were in a certain stage of devotion.
All schools of philosophy are necessary.
Each philosophy is best suited to a certain type of people. All cannot grasp the highest Kevala Advaita philosophy of Sankara all at once. The mind has to be disciplined properly before it is rendered a fit instrument to grasp the tenets of Sankara s Advaita Vedanta. The Dvaitadvaita Philosophy of Sri Nimbarka from Swami Sivananda's book All About Hinduism .