When was Super Mario on Sega? The rivalry between Nintendo and Sega was so fierce that there's no way Nintendo would have leased their mascot game to their rival.
Now to answer the thread question, I would have to know what gen and platform they're starting off on. I wouldn't recommend anything that goes too in depth. Meaning I would leave most AAA games out at the time. I would integrate them in slowly with what their interests are and go from there.