I've never got far in trying to read all of it, but I still remember some of Satan's speeches from Book 1** which I learned at school:
What though the field be lost?
All is not lost; th’ unconquerable will,
And study of revenge, immortal hate,
And courage never to submit or yield:
That glory never shall his wrath or might
Extort from me. To bow and sue for grace
With suppliant knee, and defy his power
Who from the terror of this arm so late
Doubted his Empire, that were low indeed,
That were an ignominy and shame beneath
This downfall.
And:
The mind is its own place, and in it self
Can make a heav'n of hell, a hell of heav'n.
What matter where, if I be still the same
. . .
To reign is worth ambition though in hell,
Better to reign in hell than serve in heav'n
When I was writing my SFs I prefaced each section with a quote from a poem, and for one where the main character had received crushing news I used lines from Paradise Lost:
What reinforcement we may gain from hope,
If not what resolution from despair.
** Sorry
@Parson, but in this case the devil really does have all the best lines!