Vib-Ribbon.
It's one of the coolest concepts I've seen for PSx. It's a rhythm game that loads into the console's ram, allowing you to swap the disc to one of your CDs and could generate unique levels for your songs.
While looking it up I did find there was a PS3 port, but I don't know how good that is or if it has the same custom level feature or not. I'd love a modern version that looks visually similar but allows you to either input a playlist from a service like yt or use your own local files to generate unique levels.