220px-themamasandthepapas-ifyoucanbelieveyoureyesandears#1 – The Mamas & the Papas – “California Dreamin'” – (1966)

Here it is – the song that defines the entire decade. Originally recorded by Barry McGuire, it was actually written by members of this group. This song marked the arrival of the entire counterculture movement and announced California as its home. It was a top five hit and has been used in numerous other works… it just has that ability to set you in a time and place like very few other songs can.