Easy for me: George Gershwin's "Summertime"!! I think I have around 10-20 different versions of this song and the best I've heard is by Ray Conniff, his orchestra and his chorus. From his 1960 lp "Say it with music (a touch of latin)".
Composed somewhere between 1926 (where Gershwin started to work on his "Porgy & Bess" musical) and 1935 (where it was first staged) - most likely in 1926, as it was the first song he composed for "Porgy & Bess". After 77 years it can still enchant people all over the world!