Mine is Dark Cloud. It was a PS2 launch title (or near enough to it) that was sort of a PlayStation answer to the Zelda franchise. Along with the original Spyro trilogy, Dark Cloud was by far my most-played game back in the day. It had an absolute banger of a soundtrack and a few pieces of really interesting unique gameplay including an RPG element where the primary progression system was not in your characters, but in upgrading your weapons, and a city-builder where you have to place all the people in each village near or away from various other elements in the village to meet their needs.

I almost never hear anyone talking about Dark Cloud, but extremely randomly one of the like four Twitch streamers I actually watch (all of whom are Age of Empires streamers, because that's basically all I watch these days) happens to play its soundtrack frequently on her stream as background music. So that's been really fun.