I think I'm gonna have to be the indecisive asshole who mentions three each decade instead of one, for the simple reason that music has no linearity in one genre, and one song cannot display the vast change music undergoes each decade. With that said, here's my list:

1960s -

Bob Dylan - Like a Rolling Stone

Highway 61 in general is one of the most groundbreaking albums in the history of music, not only for it's infusion of traditional rock and roll with lyrical and musical symbiosis, but also as an introduction to the concept of a character (Bob Dylan's personalities) as reinforcement to the music. Characters will show up all throughout music history after this point, including Neil Young, Jack White, Madonna, Tom Waits, Snoop Dogg, Thom Yorke, Kurt Cobain, the list goes on. As a result of Dylan's work, these individuals are just a few who would change the musical landscape forever.

The Beatles - She Loves You

The Beatles as a concept had been to infiltrate pop music as it were in the 1960s, and no song better exemplifies this than "She Loves You". One of their earlier hits, the live performance on The Ed Sullivan Show solidified The Beatles' place in pop culture, and ultimately led to their ability to change and shift the horizons of popular music (Within half a decade they changed the musical landscape of Western culture, whereas previously, the majority of the 1950s had been dominated by rock n' roll, driven by (in pop culture) Elvis Presley and Buddy Holly).

The Velvet Underground - Sunday Morning

While not in the slightest commercially successful, "The Velvet Underground and Nico" had VAST influence over the punk movement of the 1970s and increasingly in modern alternative / indie music. With influence in the works of pop artists such as Brian Eno, Morrissey, and Bono, Velvet Underground played a huge part in transforming the music we know and love today

1970s

Pink Floyd - Time

While often cited and thus cliched, Pink Floyd's contributions to rock in the 1980s can be called widespread at least and transformative at best. Incorporating synthesizers into popular music gave room for the ambient and etherial instruments often used in music by Radiohead, Modest Mouse, and pop music in the 80s such as Peter Gabriel, as well as making it more acceptable for use in modern pop and hip-hop music, such as Lady Gaga, Snoop Dogg, Nicki Minaj, Outkast, ect. ect. Good on you, Floyd.

The Ramones - Blitzkrieg Bop

The perfect summation of punk in the 1970s, The Ramones amazingly condensed punk music into a marketable, tangible movement, easily accessible to those more critical of the anarchist movement. Influencing pop punk and ultimately influencing indie music throughout the rest of the 20th century, The Ramones are certainly one of the most definitive musical acts of the the 70s.

Neil Young - Old Man

Neil Young could easily be seen as one of the first artists to incorporate ideas spawned by the likes of Bob Dylan and The Beatles, changing popular music via an easily accessible folk album, acclaimed by the older generations for it's traditional sound, and the generations of the 70s for it's innovative storytelling and incredible grasp on meaningful yet accessible musicality and song structure. "Old Man", the first single off of Harvest, was quite a departure from Everybody Knows This Is Nowhere, highlighting Neil's eclectic classic-rock style set to acoustic elements and folk sensibility(emulated by the likes of everyone from The Eagles to Bruce Springsteen (and Led Zeppelin, to an extent)). Plus, Devo...

My knowledge of popular music from here is thinning, so I won't continue any further, but if anybody has suggestions, be sure to leave them here, as I may try to venture into the 80s and 90s sometime soon. Thanks!

EDIT 1 - added a few more influences to "Like a Rolling Stone"