First you don't want to overwhelm them with details, or with complexity, I assume that you are going to have them play the game; if that is the case then keep things simple. Nothing that is going to color their experience, and bias their opinion of gaming. So as much as I love bioshock that is a horrible idea. I would also stay away from First person shooters in general, not only might the violence turn them off, but the shooters tend to be some of the most competitive games.
You also want something that isn't too hard to learn control's for, RPG are out here as well because they are overwhelmingly complex for a newbie.
If you want something that came out recently try Red Dead Redemption. The game has incredible aesthetics, a good story, and while the controls are a little complicated things like the auto aim and dead eye make the game easy enough for beginners to grasp.
If you don't mind going old school then Mario is the way to go, it is an old game but is has aged well, and the gradually scaling difficulty, lack of hard core violence, and cartooney art style help to ease people into it. Plus it is probably one of the simplest games ever made, and pretty much everyone has heard of Mario.