There is no official line, but the most famous passage of Purgatory is the invective of canto VI against Italy, which begins with "Ahi serva Italia, di dolore ostello, nave sanza nocchiere in gran tempesta," that is, "Ah, enslaved Italy, inn of sorrow, ship without a pilot in a great storm." Dante unleashes it after seeing Sordello embrace Virgil at the mere name of Mantua, and turns it against Italy, against the emperor who abandoned her and against the Church.
Among the isolated lines, three sum up the cantica. The first defines the kingdom: "and I will sing of that second kingdom where the human spirit purges itself and becomes worthy to rise to heaven." The one from canto XVII places creator and creature in the same statement: "neither creator nor creature was ever without love." And the one from Virgil's last speech gives the measure of the freedom the journey hands to Dante: "free, upright and sound is your will."
No line of Purgatory eclipses the others, as the inscription on the gate of Inferno does. The cantica is more a matter of process than of image, and so the lines that sum it up say what a man learns to want.