Final answer:
The Concerto Grosso is the instrumental form that is regarded as the greatest artistic achievement of the Baroque period, alongside Opera. It features a small group of solo instruments accompanied by a larger orchestra.
Step-by-step explanation:
The instrumental form that is perhaps the single greatest artistic achievement of the Baroque period, next to Opera, is the Concerto Grosso. It is a form of orchestral music where a small group of solo instruments (concertino) is accompanied by a larger orchestra (ripieno). This form reached its peak in the works of composers like Antonio Vivaldi and Johann Sebastian Bach, who composed famous concerti grossi.
Learn more about Concerto Grosso in Baroque music