"Attar is one of the greatest poets of the Persian language. Nonetheless, his popularity - both in Iran itself and in the West (Goethe, for example, touched on him only briefly in his West-Eastern Divan) - does not match that of Ferdowsi (d. 1020), Omar Khayyam (d. c.1132), Rumi, Saadi (d. 1292) or Hafiz (d. 1389); occasionally he is even omitted from the line of seven Persian poet-princes in favour of Jami (d. 1492). One possible reason for this is that the composition of his poetry is too artful, too complex to be effective in the town squares and teahouses, while at the same time, many of his stories and figures may seem too coarse, too folk-like and too sarcastic to be at the forefront of the high spiritual literature cultivated at courts in former times and in middle-class households today. Attar’s poetry, on the other hand, is far less stilted than that of most Persian poets but, rather, unadorned, clear and immediate. The pain it expresses is not spiritually filtered as in Rumi, far less metaphysically elevated than in Saadi, and not sublimated into pleasure as in Omar Khayyam - where Hafiz turns the earthly into the mystical, Attar strips mysticism down to its leaden, earthly foundation in order to scream his longing to the heavens."
--Navid Kermani, The Terror of God: Attar, Job and the Metaphysical Revolt
I asked my professor which masnavi (Persian epic poem) he thinks is the greatest ever written. He replied, Rumi's Masnavi (the only masnavi Rumi wrote). Shock. How can there be a masnavi greater than Attar's Conference of the Birds? (There are 4 authentic Attar masnavis; sadly, as far as I know, Conference of the Birds is the only one that has been translated into English.) Reading through Rumi's masnavi I think I am still team Attar. It's Attar's coarseness I love--he is a poet of mad saints and freaks. In Rumi's Masnavi, the absence of a frame story and the pious/didactic tone is somewhat of a barrier for me. The pieces don't quite hang together, whereas Attar's Conference of the Birds is intricately structured--there are stories within stories within stories, each bird with its idiosyncratic psychology--a narrative arc that mirrors the journey of the soul across the seven valleys. But maybe there is a difference between reading a sufi text for its poetry rather than religious instruction, I don't know.