Great thread idea. I have a lot of favorites but I can only guess at why they went unused.
There's some great unused arrangements from Castlevania, notably the Rondo of Blood version of "
Cross Your Heart" and the Rebirth version of "
Battle of the Holy." I believe you can unlock the former in Dracula X Chronicles and assign it to whatever stage you want, but I have no idea which stage it was originally meant for.
FFV also has a really good unused version of "
Matoya's Cave" from FFI. Pretty triumphant sounding so I don't think it would have been a dungeon theme. The unused tracks from FFVI suck, but "
Sea of Silence" from FFIV is great. I'm guessing the area it would have played got cut rather than the music being unsuitable because it fits perfectly with the style of the rest of the soundtrack. Or maybe it was going to be for sailing?
While I'm not a huge fan of the Actraiser soundtrack, I like some of the original FM versions and the unused "
Space Fight" is the crown jewel. It being so different from the rest of the soundtrack is probably why I like it so much and also why it was cut.
"
Prison House" went unused in every version of Ys IV as far as I know, maybe cause it was too upbeat for a jail scene. Makes for good listening though.