Chaucer was consider the "English Homer" (http://www.wwnorton.com/college/english/nael/middleages/welcome.htm). That means that his writing did more for the English language (to solidify it) than anyone else before his time. With the invasion of William the Conqueror in 1066, everyone was required to speak French; that's why English has so many Latin and French words today. It is interesting to note that the Norton Anthology introduction on the above-mentioned website also says:
"One of the results of the Norman Conquest was that the structure and vocabulary of the English language changed to such an extent that Chaucer, even if he had come across a manuscript of Old English poetry, would have experienced far more difficulty construing the language than with medieval Latin, French, or Italian. If a King Arthur had actually lived, he would have spoken a Celtic language possibly still intelligible to native speakers of Middle Welsh but not to Middle English speakers."
Chaucer was a great poet. I think that when a writer's works are still loved and studied years later, that shows a real contribution. Also, Chaucer is foundational to other English works that came later. I think that The Canterbury Tales still attract people's imaginations the most.