I am free-associating on the words "coolest song".
In 1973, Johnny Mercer composed the theme song to Robert Altman's movie The Long Goodbye. Altman and Mercer collaboratively came up with the fairly unique approach of using a single theme in various vocal and instrumental incarnations and fragments throughout the soundtrack. The various song performances are nearly as cool as Elliott Gould's Philip Marlowe. I have the CD of the soundtrack!
The performance I prefer is by Clydie King performing the song "The Long Goodbye":
Here is another lovely performance, this by Irene Kral and the Dave Grusin trio with the same song:
In 1996 Mono (vocals by Siobhan de Mare) performed "Life In Mono" from their Formica Blues album:
Justyna Steczkowska sings her song "Dziewczyna Szamana" on her amazing debut album circa 1996:
Justyna sings her song "Oko za oko" on the same album:
Here is a song recorded in 2008 by Kasia Nosowska, covering famed poet/singer-songwriter Agnieszka Osiecka's 1970 composition "Zielono Mi" (music by Jan Wroblewski):
In 1979, The Flying Lizards (cool chic pouty vocals by Deborah Evans-Stickland) covered the song "Money" previously made popular by The Beatles:
Edda Magnason covered Monica Zetterlund's version of Dave Brubeck's "Take Five" for the soundtrack of a movie biography of Monica in which Edda acts the part of Monica:
In 1963, Dick Dale and His Del-Tones played surf guitar on "Misirlou", which saw a revival of popularity in 1994 on the soundtrack of Pulp Fiction: