I'd second Portal. Really I wouldn't do it because there's too many different types of engagement and no game can cover them all. Show them any game and you'll probably create misconceptions (for example Portal is very puzzle focused with clearly defined levels and teaches you how to do one thing per level (ish)). The problem with Portal is it involves navigating and aiming in 3D space with timing and jumping(in first person!) which would be difficult for people.
But on the positive side Portal is
1. Widely acclaimed
2. Has a setting
3. Has a story/comedy
4. Teaches the person what to do
5. Builds up on that idea so they can see how things go from simple starts to much wider complexity.
6. Is 3D (yeah this is also the main negative)
7. Is fairly short
The problem with Mario games is that 2D gaming has become fairly uncommon in the wider stream and it's purely challenge based with very few of the other types of reasons people play games. And if we change the condition so it's a person who lives in the real world but has never touched a game or discussed it with someone, it would reinforce the stereotypes (scores, points, clearly defined levels, chippy music, excuse plots, 2D, linear) These aren't bad things but they don't correspond to what most people play nowadays.
EDIT EDIT: I'm probably saying this because I started playing game really during the PS2 era (although I'd had some experience with an N64) and so for me I don't associate first with the older games, in my mind they end up being the precursors to the games that were around when I started playing.
But I think games are really too broad. I couldn't answer the question if it was what film would you show a person to show them what films are and games are much broader and more complicated
EDIT:
PieBrotherTB said:
Actually I like all your suggestions, From Dust would be a really good one. Maybe even better than Portal. And its introducing a mechanic no-one is familiar with so the learning curve probably wouldn't be so steep. And there's some sandbox elements it introduces which would be a nice way of introducing the concept of games where there's something you can explore and play around with