The four books listed are arguably the four most important books in Paradise Lost, and any of them could well be chosen as the most significant. Books I and II open in medias res with Satan and the fallen angels dramatically awakening in hell after being defeated by God. Book IX is the dramatic climax of the story in which Satan successfully tempts Eve to eat the forbidden fruit. Book XII is the high point of the ending denouement as Adam and Eve leave paradise and God lays out his plan to vanquish Satan, while Adam has finally learned the lesson of obedience to God before all else.
Given this choice, I would chose book I as the most significant. This is because it sets the whole epic into motion and offers the most compelling possible vision in all of literature of a fatally flawed Satan as a worthy foe for God. First chapters are often very important, and this is no exception.
Book I begins by laying out the subject of the epic poem, ending with the promise that it will "justifie the wayes of God to men." From line 45 on, the action starts, as Satan and his minions are hurled headfirst "flaming from th' Ethereal Skie" to hell. Satan, the opponent who will clash with God, is unvanquished despite his recent defeat and describes himself as an undaunted fierce rival, saying:
What though the field be lost?
All is not lost; the unconquerable Will,
And study of revenge, immortal hate,
And courage never to submit or yield:
And what is else not to be overcome?
We then are thrown into the middle of gripping action as we witness Satan break his chains and rise from his pool of fire and brimstone:
Forthwith upright he rears from off the Pool
His mighty Stature; on each hand the flames
Drivn backward slope thir pointing spires, and rowld
In billows, leave i'th' midst a horrid Vale.
He then unfurls his huge wings and flies. He states defiantly that although he is in hell:
The mind is in it own place and in it self
Can make a Heav'n of Hell, a Hell of Heav'n.
Satan also argues that it is "Better to reign in Hell, then serve in Heaven." The drama continues to move and build without stopping, as Satan gathers his minions around him. They immediately build Pandemonium, a parody of God's celestial city, and begin to plot their next actions.
The powerful and compelling character of Satan as outlined in this book has long captured the imagination of readers, with the Romantic poets in particular seeing Satan as the true hero of the poem. Whatever the case, with its rich description, unforgettable—and much quoted—antagonist, and non-stop action, this is a book that pulls us into the rest of the epic.