The 1963-1967 period was, in my opinion, the highest jump of popular music in all its history. During those years there were some key songs that opened new doors and pushed forward music boundaries with new sounds.
The next list includes the songs that I selected as the most revolutionary of each year from that period. I limited my selection to songs that actually were being played on the radio, having a literal effect on the air of the time.
1963 - I Want To Hold Your Hand (The Beatles). Other songs like "Please Please Me" or "She Loves You" could also have been mentioned; but this song opened the American door not only for the Beatles but also for the whole British Invasion.
1964 - You Really Got Me (The Kinks). This song could be considered as the first notes of hard rock. It's probably the heaviest rock/pop recording done until that moment, beyond the amateurish performance.
1965 - My Generation (The Who). Not only the sound was wild for the time, the message was also strong: youth people started to speak loud.
1966 - Eight Miles High (The Byrds). One of the earliest truly psychedelic songs, though "Shapes Of Things" by the Yardbirds should also be mentioned. The song is characterized by free and random guitar playing, intricate drumming, hypnotic bass and elevating vocals.
1967 - Somebody To Love (Jefferson Airplane). One of the original acid rock songs, along with Jimi Hendrix's "Purple Haze". This is raw psychedelia, without the need of heavy studio experimentation. This new sound was exciting and wild, including blazing guitar solos and thunderous bass.