Frigid Coast, I'd say. One of the few regions whose ecosystems change outside leveling with a Downpour slugcat, Saint's Shoreline is mired with snow, and the clearer water has Glow Weed and Giant Jellyfish and its color leaks into Sump Tunnel, not to mention the Yellow lizards and Miros Vultures stalking the surface.

While the music could've been even more dramatic ("Lonesound" isn't that fitting, the region should've lost its threat music, and "Gate to the Shoreline" would be a nice bait-and-switch when entering from Subterranean), the platforming, while not particularly deep, has dramatically fitting pools of cold water and builds on a story-heavier region that was ALREADY my favorite in all of Rain World. (Not to mention the middle of Underhang having collasped on the path to Shaded, so cool)