I used Samurai X: Trust and Betrayal. It's a completely serious side of anime that people don't tend to see, especially since if by chance they have seen the series that goes with it, which was aired on Toonami back in the day, it was much less mature (lots of fighting but no killing). The only downside to using this is that the you can't appreciate the violence of the main character without having first seen him as the pacifist he is in the main series. However, since its a prequel, it works pretty well. Lastly, its a movie, which is easier to get someone to sit down and watch.