1740's
Handel's oratorios, Bach's Musical Offering and Art of the Fugue, Scarlatti's sonatas.
In the 20th century... I'd say the 1950's. The electric blues of Howlin' Wolf and Muddy Waters, plus bebop reached its zenith thanks to the innovations of Bird, Dizzy Gillespie and Miles Davis and the electronic music of Raymond Scott and Stockhausen. Plus Richard Strauss's Four Last Songs. I'd say even the Beatles would have a hard time beating that.