What's the most "mature" Nintendo-developed game?

[GooseGaws]

To be more specific, I want only games that are completely developed by Nintendo's internal teams. That excludes Retro, Silicon Knights, n-Space, etc. The first Star Fox as well as 64 were developed in-house, while Adventures was by Rare, Assault was by Namco, and Command was by Q-Games. Super Metroid is definitely right up there; I remember being shocked by the dead bodies of the scientists in the lab. However, I still think Star Fox 64 surprised me the most, considering the tone of the previous game and its decidedly "cutesy" cast of characters.