Blue Moon
You Are My Sunshine
Yesterday
Here are some songs I find are recognized and generally appreciated by audiences of many generations. You didn't specify "folk," so I'm not sticking to that category.

Genie

Oh, What A Beautiful Morning
La Bamba
Proud Mary
Tennessee Waltz
Singin' In The Rain
Wild Mountain Thyme
Puff, The Magic Dragon
Any Time
City Of New Orleans
Shine On, Harvest Moon
Home On The Range
Take Me Home, Country Roads
Loch Lomond
Danny Boy
Wabash Cannonball
Bye, Bye, Blackbird
Scarborough Fair
Wildwood Flower