It just HAS to be Dylan. He defined the angst of the generation in a spectacularly special way. Take the music away and he's still the poet laureate of the generation. Poetry gets its power through metaphoric ambiguity and Dylan had truck loads of it.
After Dylan, I'd say John Lennon. When you're widely listened and accepted the influence and the power that your music has for change is so much the greater for it.
Very, very few songwriters (self included)have escaped the influences of the above mentioned, whether they know it or not.
Honorable mention should be given to Woody Guthrie though. Albeit, he mostly belonged to other decades, it all pretty much comes back to him more than it does anyone else. That's what I think.