One of the problems with the early numbers in this sequence is realizing the good stuff I've missed. For 20 I could have had "20th Century Boy" by T. Rex, but "20 Flight Rock" by Eddie Cochran is still a great so that's fine. I have the songs mapped out to 41 at the moment with a few scattered between there and 59, and the original premise for 21 was "21" by The Eagles from the album "Desperado".
Coming home however the perfect, for me, 21 song came to mind, six and a half minutes of monstrous jazz rock that the Rolling Stones had to follow in Hyde Park in 1969, not other than "21st Century Schizoid Man" by King Crimson featuring the psychotic and psychedelic lyrics of Pete Sinfield, Greg Lake on vocals and Bob Fripp on guitar. This is still an incredible piece and it always amazes me. I bought the album "Court Of The Crimson King" on DVD to listen in full surround sound , and though it's close on fifty years old it sounds stunning. I managed to find the Hyde Park broadcast but the album should be in your collection.
Time for bed now, though this is not music to fall asleep to.