Unique can be taken in multiple ways.
Thinking through my albums I think one of the most unique songs is Ning Feng’s The Last Rose of Summer on his Solo album. It’s a mind-blowing difficult solo piece that I also find to be almost "easy listening" which often isn’t the case for music that is so technical.
One of my absolute favorite classical pieces is Liszt: Schwanengesang, S560 - Serenade that I have on a Vladimir Horowitz DG recording. When this song plays it catches my attention and I often start it over multiple times before moving on.
My first thought reading this thread was Pachelbel's Cannon...