Claudio Monteverdi: Lamento d'Arianna ("Lasciatemi morire") (1623). The only surviving music from Monteverdi's second opera, L'Arianna, the Lamento is sung by Ariadne, distraught at being abandoned by Theseus on Naxos. This lament is composed with such mastery and emotional power that the possibility that L'Arianna was Monteverdi's best opera is a matter of lament in itself.
Ariadne sings here in pain, anger, and despair, but ultimately confirms her love for Theseus ("it's my tongue that's spoken, not my heart"). It survives, presumably close to the original operatic version, as a solo aria with continuo (published in 1623) as well as in two arrangements by the composer, the first, the dramatic center of the Sixth Book of Madrigals (1614), is startling at all moments in its transfer from monody to an ensemble of five voices (to risk a bit of psychologizing, when the text is sung by an ensemble, it is as if we were no longer listening to Ariadne's voice, but thrust directly into her head, raging with pain and conflict), while the second recycles the music of the monody but sets a new, sacred Latin text, "Pianto della Madonna" for the collection of Selva morale e spirituale (1641) in which the Virgin laments her abandonment by her son, interesting for both the implied synchretism between the mythical and religious figures as for the implied equation of two very different kinds of loss. (Or are they so different? Men leaving women is as old a story as any.)
I have a weakness for, no, I am totally lost to the lament style, and I am constantly astonished to encounter the style as it branched out into strikingly different contexts: as Purcell's Dido sings over and against the regularity of the ground bass, for example, or in Heinrich Isaac's devastating choral setting of Poliziano's lament on the death of Lorenzo de' Medici, "Quis dabit Capite meo aquam? The essence of the style, as far as I am concerned, is its very free voice leading — that is to say, free in the treatment of the approach to and resolution (or lack thereof) of dissonance — with a rather wide open tonal vocabulary, features which were, and are, essential to opera.