Which is the best work from Goethe?

The most important of Goethe's works produced before he went to Weimar were Götz von Berlichingen (1773), a tragedy that was the first work to bring him recognition, and the novel The Sorrows of Young Werther (German: Die Leiden des jungen Werthers) (1774), which gained him enormous fame as a writer in the Sturm und Drang period which marked the early phase of Romanticism. Indeed, Werther is often considered to be the "spark" which ignited the movement, and can arguably be called the world's first "best-seller". During the years at Weimar before he met Schiller in 1794, he began Wilhelm Meister's Apprenticeship[42] and wrote the dramas Iphigenie auf Tauris (Iphigenia in Tauris),[43] Egmont,[44] and Torquato Tasso[45] and the fable Reineke Fuchs.[46]To the period of his friendship with Schiller belong the conception of Wilhelm Meister's Journeyman Years (the continuation of Wilhelm Meister's Apprenticeship), the idyll of Hermann and Dorothea, the Roman Elegies and the verse drama The Natural Daughter.[47] In the last period, between Schiller's death, in 1805, and his own, appeared Faust Part One (1808), Elective Affinities (1809), the West-Eastern Diwan (an 1819 collection of poems in the Persian style, influenced by the work of Hafez), his autobiographical Aus meinem Leben: Dichtung und Wahrheit (From My Life: Poetry and Truth, published between 1811 and 1833) which covers his early life and ends with his departure for Weimar, his Italian Journey (1816–17), and a series of treatises on art. Faust, Part Two was completed before his 1832 death and published posthumously later that year. His writings were immediately influential in literary and artistic circles.[47]

Assuming that literature is not an exact science and that works cannot be divided into objectively assessable works, such as good, bad or excellent, and that Goethe created a large number of pieces, I will answer the question as follows:
The most important works for Goethe are Goetz von Berlichingen and The Sorrows of Young Werther, since these works gave him initial fame in the Sturm und Drang period.