Paul Simon
Bob Dylan
Tom Petty
Willie Nelson
If you're using objective metrics, like albums sold, radio plays, authorship of songs that are permanently in the American consciousness, people who are/were bigger-than-life icons (e.g. non-musical magazine covers, mentions in non-entertainment media like NY Times), songwriters whose songs are frequently covered... in some order, that's probably your top four, at least of the last 50-60 years, right?
I don't know who the next two are... maybe Prince and Springsteen.
Johnny Cash was certainly iconic and might get into an objective-criteria modern top 10.
Anyway, that's not *my* list, but my guess at seven of the top 10 if you used some reasonable and objective metrics *and* kept it post-WWII. And no one would click on a list like that because it's boring.
I'm probably forgetting someone obvious and will wish I could go back and edit this...
It gets a lot harder to measure guys that didn't live with modern media... like Hank Williams or Robert Johnson... whose faces most people wouldn't recognize but whose influence was massive. It's also harder to compare guys who wrote in styles that have largely faded from pop culture (or at least faded relative to 21st-century tastes) - e.g. George Gershwin, Scott Joplin, John Philips Sousa, Duke Ellington, Chuck Berry.