Sealed Gods and Awakening of the Earth Spirits are two of my favorites.
On the one hand, Sealed Gods from Touhou 10 is a relaxing song that expresses serenity, evoking the beautiful landscapes of the Youkai Mountain. Also, those sounds of falling raindrops, I think, makes it very immersive.
On the other hand, Awakening of the Earth Spirits from Touhou 11 is much more energetic, it feels like an invitation to adventure and has a Japanese touch that fits well with the folklore of the underworld.
The highest point on the mountain and the lowest point in the subterranean both fascinate me.