"Standards" for what country?   Even within "folk music" that varies a lot.   E.g., what "every folkie knows" in the US (such as This Land Is Your Land) is not the same as what the folkies in the UK "all know" (such as maybe "Carrick Fergus.")   

I could list a few songs that are petty much sure-fire sing-alongs in folk groups in the US. Is that what you're asking? Or I could list my "top 20 standards" for retirement community gigs in the US. That'd be a different list.