If by "great" you mean influential, then definitely India's epics, the Ramayana and the Mahabharata would fit the bill IMO.
Wikipedia has this to say about Mahabharata:
> The Mahābhārata is the longest epic poem known and has been described as "the longest poem ever written".[7][8] Its longest version consists of over 100,000 śloka or over 200,000 individual verse lines (each shloka is a couplet), and long prose passages. At about 1.8 million words in total, the Mahābhārata is roughly ten times the length of the Iliad and the Odyssey combined, or about four times the length of the Rāmāyaṇa.[9][10] W. J. Johnson has compared the importance of the Mahābhārata in the context of world civilization to that of the Bible, the works of William Shakespeare, the works of Homer, Greek drama, or the Quran.[11] Within the Indian tradition it is sometimes called the Fifth Veda.