Yesterday is the most covered song of all time. There has to be 10 great versions amongst the 3000 covers.
Summertime from Porgy and Bess has to be up there as well for a song with lots of great covers and unique takes.
Ray Charles did, IMO, the best version of "Yesterday"
"Summertime" is a GREAT pick. Here's my favorite version:
https://www.youtube.com/watch?v=XWxYx9mmr7U
It's also the best lip-synching I've ever seen.
Love that one, never heard it before. Just some of the great versions are Billie Holiday, Gil Melle, Charlie Parker, Janis Joplin, the Zombies, Miles Davis, Al Green, Louis Asmtrong/Ella Fitzgerald, Annie Lennox, Sam Cooke, Coltrane, George Benson, Duke Ellington, Chet Baker and Doc/Merle Watson.
It's one of the greatest American songs ever written and lends itself so well to so many different interpretations. No matter what, the Gershwin song is always haunting.