What an impossible question. But just for fun and off the top of my head, and in categories, here's a few I clas as best
Country Music - Blue Side Of Lonesome, Cold Cold Heart
Folk Music - Dirty Old Town, The Band Played Waltzing Matilda, Peggy Gordon
Irish - The Rare Ald Times, The Town I Loved So well, Slievenamon,Singing In Kilkenny
Pop/rock Satisfaction, Yesterday, In My Life
Opera - The Aria from Carmen