Depends on the person. If they like a good old-fashioned army sim, COD 5. A fan of horror films, resident evil games. If they're young at heart and enjoy cartoony things which are just plain fun, then Super Mario games are the way to go, plus brawl. If they want a good story I won't hesitate to direct them to either Fable or Final Fantasy X. Finally, if they want to shoot things and make stuff explode to release stress, I'd run with a therapy course of Saints Row 2 and Halo games.
EDIT: And I think everyone should play the old platformers and brawlers. Streets of rage 2 and Ristar are in my opinion prime examples of the best of the 2D era, as is Golden Axe.