The Rite of Penance indicates that for Form I, the Scripture reading is optional but it gives no less than 113 recommendations before instructing that even other choices may be made, even with the collaboration of the penitent:
43. Then the priest may read or say from memory a text of Scripture which proclaims God’s mercy and calls (the penitent) to conversion.
A reading may also be chosen from those given in nos. 72-83 and 101-201 for the reconciliation of several penitents. The priest and penitent may choose other readings from scripture.
Nos. 72-83 are short readings, two Old Testament, and ten NT (five of which are Gospel readings).
Nos. 101-201 are longer readings sectioned off by liturgical function: thirty-two readings from the Old Testament (average length about ten verses), fifteen psalms with refrains (ten of these are suggested in their entirety), twenty-eight from the New Testament (five to twenty-four verses long), and twenty-six Gospel passages.
I’d say that most liturgists and pastors don’t give these passages consideration for form II–they’re recommended for communal reconciliation as well. The Old Testament passages in particular deserve a glance.
Any favorite Scripture passages amongst our readers: confessors, penitents, or liturgists?