What the hell is mature mean, anyway? Are we just talking about whatever stereotypical adult themes are, and the level of tact with which they're handled?
For non-RPGs I want to say The Sea Will Claim Everything or Photopia -- especially Sea; Photopia's arguably a bit too emotionally manipulative -- but these are both barely games.
Although if I wanted to pick a game that had themes that, like, grew with me? Maybe Earthbound for the RPG?