Each and every one of these tracks is supposed to be a personal choice. This one, especially, lends itself to personal choice, as we all have our own definition of underrated. To me, it means a music track that doesn't get the recognition it deserves from the fanbase; it doesn't top many favorites lists, nor is it remixed as much as it should be.
Old Yuanxion. Or Old Distant Hermit, of you must spell out the translation. Seiga's theme from Ten Desires was a gem that I uncovered several years ago, and it's stuck with me all this time. It could easily have qualified for music that gets stuck in my head or for music I don't get tired of, but this day's entry felt right for it.