If you are set up for vinyl, Arthur Lyman: Taboo on HiFi Records. You need the original 1958 stereo pressing, still easy to find anytime for $5. His style (vibes) is masterful but very restrained, and he recorded in the hemispherical aluminum Kaiser dome, which provides additional ambience. Lyman's other work for HiFi ca. 1958-1959, Bwana A, Hawaiian Sunset, Legend of Pele, Bahia, is equally Old School chill. You will need a martini and a plushly-upholstered chair.
I agree with several of the above more recent artists and titles, but this is the chilliest that I get.
