Saddest and darkest Mario game is definantly Super Paper Mario. The saddest and darkest game/book/movie of all is/WILL be Harry Potter and the Deathly Hallows :cry: . Although, if you want I'll give you want PM you a list of people who die.
Luigi's Mansion was pretty creepy. I think I might consider TTYD the saddest Mario game (or at least the most profound), but I don't know why and I'm not willing to think up a reason. I probably shed the most tears during SMRPG's ending, but those were tears of joy.