The term "alternative rock" is pretty broad and I for one don't want to debate its proper meaning. Suffice it to say, I tried to pick one song from each era in which a major "alt rock" movement existed, such as I can personally identify and define one.
1970s: Joy Division, "Transmission"
1980s: Dinosaur Jr, "Little Fury Things"
1990s: My Bloody Valentine, "When You Sleep"
2000s: Black Rebel Motorcycle Club, "Rifles"
2010s: Wire, "Doubles & Trebles"
Cool question, got me digging through a lot of awesome stuff!