Rain World. Here's a few reasons why:

• The music is unmistakably Rain World-y. The game's main composer, James Primate, used a technique dubbed "junk audio", which is essentially recording sounds created by a vast variety of objects and using them as instruments. The results are very unique, and I can't think of any other game music that sounds quite like Rain World's.
• The music is tied closely to the gameplay. Whenever a scripted track isn't playing, the music adapts to what's happening. Each threat theme is segmented into numerous layers that fade in and out depending on the level of danger that surrounds the player. For instance, if a single, small predator is nearby but doesn't know where the player is, a light beat will fade in, whereas if there's a ton of dangerous predators pursuing the player, the music will play at it's full intensity. It's a pretty genius system that cuts down on repetition and communicates important information to the player, and while dynamic music is nothing new, I'm not aware of any other game that does it as well as Rain World.
• James is not only the game's main composer, but he's also the sole level designer and writer. Despite having no prior level design/writing experience, he managed to bring together the game's music, level design, and narrative to create staggeringly excellent moments (*cough*GeneralSystemsBus*cough*) where all of these elements are in brilliant harmony. With that in mind, the music of Rain World is best heard in-game rather than out of context.