A jazz standard (as opposed to, simply, a standard) is a song written by a jazz musician that gets covered so frequently that it enters into the repertoire of improvising musicians on a permanent basis. It doesn't happen all that often (most jazz repertoire still comes from what is called The Great American Songbook - Broadway and Hollywood, in other words), but pieces such as "All Blues" and "Nardis" by Miles Davis, "Dolphin Dance" by Herbie Hancock, and "Round Midnight" by Thelonius Monk are some well-known examples. One jazz standard that I've heard many musicians call their favourite tune of any kind is "Falling Grace", by the great bass guitarist and composer, Steve Swallow. It is extraordinary in its modulations (i.e. key changes). Depending on how one analyzes it, it could be considered as changing its key center fourteen times (!) in a twenty-four measure song. Yet there is logic and consistency throughout. The piece was originally written for the piano giant Bill Evans, and his versions are the ones that I'll recommend at the top of the list, but search Gary Burton Quartet fea. Pat Metheny - Falling Grace in YouTube to hear Swallow (followed by Burton and Metheny) solo brilliantly on a brilliant song.