Aside from Beowulf, Paradise Lost is the first poem of epic proportions to be published in England.
It is considered a classic for several reasons, one of them being the enormity of the task in rewriting the creation story of the Bible. The language is beautiful and captivating, and he tells the story from different points of view--Adam, Eve, the Serpent, etc.
In addition, as with any "classic", Milton asks the important and difficult-to-answer questions that others may shy away from due to their controversial nature. Some of these are listed in the enotes summary, and are as follows:
If the world was created by a good, just, and loving God, why is there little evidence of goodness and justice in the world? What does it mean for humankind to be created in the image of that God, and how does humanity endure in a fallen world?
Follow the link below to read more about Paradise Lost.