The consensus of Internet searches in 2018 is that "Yesterday" is the most covered song of all time. This is supported by The Guinness Book of World Records which states that Yesterday has been recorded by more than 3,000 artists.
Some of those Yesterday cover artists are Marvin Gaye, Ray Charles, La Lupe, Merle Haggart and Willie Nelson, Aretha Franklin, Joan Baez, Liberace, Bob Dylan, Placido Domingo, The Supremes, Adam Levine, Frank Sinatra, Elvis Presley, En Vogue, Boyz II Men and even Daffy Duck.
See SecondHandSongs for over 700 cover versions of Yesterday.
Yesterday, Written by John Lennon, Paul McCartney
Recording date June 14, 1965
Released on Help! Album, August 6, 1965, Parlephone/EMI
Yesterday, Side B of The Beatles, Dizzy Miss Lizzy/Yesterday
Release date 1965
Parlophone, 45 rpm
Yesterday, Side B The Beatles, Act Naturally/Yesterday
Release date 13 Sepember 1965 (US); 08 March 1976 (UK)
Capitol Records, 7-inch single 45 rpm
Recorded 14 June 1965 EMI Studios, London
1995 Rhino Records soundtrack which includes Daffy's cover of Yesterday - Click on album cover to listen