L'incoronazione di Poppea
Monteverdi was an experienced composer when he wrote this and the opera has some of his best music.  The orchestra is only a small group of instruments: strings, recorders, cornetts (an old kind of trumpet), percussion, harp, lutes and gambas.  The music does not divide sharply between recitative (narrating music) and aria (set songs), but flows gently from one type to the other.