Dream's Deep from Mario & Luigi: Dream Team.
Like, just seeing all those messages about Luigi makes it such a sad moment. Like, you can just see the love he has for Mario, but at the same time you see all his insecurities, all his emotions of being stuck in his brother's shadow, and how much he wishes to be like his brother.
Not to mention the fact that the music and the design of the area, especially with the Luigi Shadows, make you get a shiver down your back. It really is such a deep moment, hence the name. It really was a pretty dark and depressing area of the game, especially compared to the rest of it.