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.