Im a 50 year old duel US/Canadian citizen, born and raised in the States.
Like most countries Im sure, I think itd be impossible to pick one folk song. Certainly in the US or Canada, depending on how you define folk, there isnt one folk tradition to be the source of any one song.
In the modern world, if were going for a song everybody knows, it would probably be a commercial jingle or tv theme song. Those change over the generations though, so kids today dont know the Brady Bunch theme (heresy).
By coincidence, today is the 90th anniversary of the first recording of the Cajun national anthem Jole Blonde, so theres one candidate, for one folk within the patchwork.
History of the song:
http://earlycajunmusic.blogspot.com/2014/08/ma-blonde-est-partie-breaux-brothers.html
After studying the various threads of popular music in North America, I dont think any song could rise up. The older traditions arent widely practiced, even if theyre lively in their own subcultures. Different ethnic and cultural groups maintain their own favorites, but many of them arent well known beyond there.
The idea of a unified national culture is at odds with the struggle to preserve individual traditions against the supposedly insidious spread of commercial popular music.
Personally, Id vote for Louie, Louie, since Im from Washington State, but the Governor vetoed the citizens choice, leaving it up to the people to keep the memory alive.
Louie Louie: almost the State song of Washington