My best smart playlist I had in iTunes was the one that included the top 50 of all my music with 3 stars or more (one star was not music, two stars was music I would never listen to randomly, and anything above would be actual music rating), sorted by fewest plays. I would then put that playlist on random, so music I haven’t heard a lot, like newly added music, would be in heavy rotation in the start until the play count started to match my other music.