I pick Satorl Marsh from Xenoblade Chronicles.
PART 1: THE VISUALS
Honestly, Satorl Marsh is a fairly bland-ish area in the day. Nevertheless, at the same time it does what many swamp levels struggle to do and at least look aesthetically pleasing. Additionally, while it may lack the grandiose impression of Gaur Plains, it makes up for it by incorporating ruins that add in a sense of mystique and adventure.
Night, however, is where it truly shines. I don’t really need to say why, I can just post an in-game screenshot of it and let it speak for itself.
There’s just a mysterious and amazing quality about Satorl Marsh at night that really draws you in. In a way, it could be considered one of the most magical locations in the game, if not the most.
PART 2: THE MUSIC
Let’s begin with the day once again.
While it conveys the theme of “swamp level” fairly well, at the same time it also instills a sense of mystique and eerie calm that many swamp levels struggle to handle well. Already, this place succeeds where other swamp levels tend to fail by sounding pleasing and relaxing to the point where you may just want to stay a bit longer to listen to the music… and given the general flavor of “bleh” swamp levels usually hold that makes you want to get it over with, that is no small feat.
As for night…
Is this even a swamp level theme
I’m serious, Satorl Marsh’s night theme is downright amazing. It captures the mystique perfectly, and practically defines the area as something almost otherworldly even in-universe. About the only thing debatably holding it back from “best song in the soundtrack” is that it’s also in the same soundtrack as “Confrontation With the Enemy” if we’re speaking about the somewhat relaxed, mystical pacing of the song alone.
PART 3 - THE MAP
Satorl Marsh does an excellent job of making you want to explore and see every part of it, as Xenoblade’s generally quite successful with. This is very important, as it is a swamp level. I place emphasis on this because generally, swamp levels are a real bog that the player just wants to get through so they don’t have to deal with it anymore. The fact that it’s such an amazing location is a testament to how it sets, if not outright raises the bar for what an interesting swamp level should be like.
That isn’t to say that it’s just “because it’s a Xenoblade map” either, which are generally known for being quite visually interesting while maintaining a consistent theme. I’ll be honest, the game has some real stinker maps like Eryth Sea, which while visually interesting isn’t especially great beyond “omg teleporters” and really lacking in terms of exploration in a game lauded for its huge environments. There is also Sword Valley, which while interesting being that you’re walking on a dead god’s blade that became a warzone, it’s honestly a fairly dull location visually and doesn’t especially stack up to its peers in that regard.
tl;dr: Satorl Marsh is a strong contender for one of, if not outright the best location in Xenoblade thanks to its appearance and how it shifts based on the time of day, the incredible music, and great map design that makes the player want to linger around and explore its corners.