•Why it earns a slot
Why it earns a slot: Milton wrote it to defend God and ended up writing the most sympathetic account of rebellion in the language, William Blake's line that Milton was "of the Devil's party without knowing it" is a debate this poem still wins on its own terms.
Milton set out to "justify the ways of God to men" and produced the most persuasive villain in English literature instead.
This distillation is written from the freely available original, which is always the better read when you have the time: Project Gutenberg.