Adi Shankaracharya (788 – 820 CE) :

• One of the mystic Bhakti poet-saint leaders who gave a new orientation to Hinduism. He was born in Kaladi in Kerala. He propounded the Advaita (Monism) philosophy and the idea of Nirgunabrahman (god without attributes). In Advaita, the reality of the world is denied and Brahman is considered the only reality. It is only Brahman at its base that gives it its reality. His famous quotes include, ‘Brahma Satyam Jagat Mithya Jivo Brahmatra Naparaha’ meaning, “The Absolute Spirit is the reality, the world of appearance is Maya” and ‘Ekameva Adviteeyam Brahma’ meaning, “The absolute is one alone, not two”. He laid emphasis on knowledge (gyan) as that can alone lead to salvation. Upadesasahasri, Vivekachudamani, Bhaja Govindum Stotra are some of the works authored by Shankaracharya.
  
• Ramanuja (1060–1118):

• Ramanuja preached Vishista Advaitavada (qualified monism). He established Vaishanavism on a sound foundation. According
•  to Ramanuja, the way to Moksha lies through Karma, Gyan and Bhakti. The performance of duty without any selfish motive purifies the mind. This makes meditation of the Jiva on itself possible. Through such meditation the Jiva acquires knowledge of itself as dependent on God. Then it begins to love God and contemplate Him. Bhakti involves meditation just as Gyan does only in Bhakti.
• According to Ramanuja, Brahman is an entirely personal god and is considered to be Vishnu or one of his avatars. He believed that Vishnu has created the world out of his love for humans, and he also controls the world at every step. He also held that Vishnu has all the qualities of a personal god – omniscient, omnipotence, etc. The difference between Dualism and Vishista Advaita is that “mankind enjoys higher status than in pure dualistic worship and is nearer to God”. In Vishista Advaita, both the world and Brahman are considered equally real; they are not considered to be two separate entities as in Dualism.
• It is true that he did not oppose the ancient practice of restricting the privilege of reading the Vedas to the high classes, but he advised others to follow the path of complete surrender having absolute faith in the preceptor. He authored Sribhashya, Vedanta Dipa, Gita Bhasya and Vedantasara.

Nimbarkara:

• He was the disciple of  Ramanujacharya
• He was the first to introduce Radhamadhav cult which was centered to the worship of Radha and Krishna. He advocated the principle of self-surrender and worship of Krishna and Radha.He founded the philosophy called Dvaita Advaita. It was centered on the balance between Advaita and Visishtadvaita.There are a large number of his followers in Uttar Pradesh.

Madhavacharya (1238-1317 A.D.):

• He ranks with Shankar and Ramanuja as one of the three principal philosophers of the Vedanta System. He propounded the philosophy of Dwaita or Dualism. He was initiated into sanyasttva by Achyutapreksha. At the time of initiation, he was given the name Purnaprajna. It was also Achyutapreksha who gave him the title ‘Madhva’ by which he was more famously known.

Dvaita Philosophy: It considers Brahman and Atman as two different entities, and Bhakti as the route to eternal salvation.

Vallabhacharya (c. 1479 – 1531 CE)

• He was also known as ‘Achinitabhadra’ He was the first to promote Krishna Bhakthi on the basis of Maha Bhagvad Purana.He founded the philosophy of Shuddha Advaita which believed in absolute union.He prescribed Pushtimarga as the means for salvation (extreme devotion to God).