In classical.........
I'm a huge fan of 1850-1910, but I feel the apex is 1900-1910 followed by 1880-1890....it's not a coincidence that these two decades, very concentratedly focus on the greatest works by my two favorite composers.... Mahler and Brahms respectively
In Jazz
1940-1950 due to the birth bop and cool jazz, but the 50s is also very great for jazz
For Pop and Rock
1960-1970, and more specifically 1965-1975..............this period is very unique because it is the only period in history which I can think of where the biggest sellers were arguably the most artistic........The Beatles, Bob Dylan, The Beach Boys, Simon & Garfunkel, The Rolling Stones, Led Zeppelin, Pink Floyd, David Bowie, Neil Young, Genesis, Yes, The Who, The Doors.......all their finest work emerges from this period, Bruce Springsteen, The Velvet Underground, Joni Mitchell, Marvin Gaye, Otis Redding ,Aretha Franklin........it is truly a very special period for music....not everyone there listed was a million seller, but the point is...........there were two venues of music at the time AM Radio and FM Radio.........FM radio was the platform that all serious collectors went to and its today the majority of what is remembered from that period.