Depends who I am recommending to. I will assume it's an average person with interest in a wide group of games and not just HD shooters. On that basis 1. Advance Wars Dual Strike 2. Mario Galaxy 3. Fallout 3
Good point, and good question overall;
If I'm reccomending to you someone who's never even seen a video game, I'd reccomend Super Mario Bros (as an introduction), Wii Sports (for easy to pick up competitive multiplayer), and Halo 3 (not neccesarily because it is great, but because it is one of the most complete packages on the market today, and could show them the extent of what is possible with the medium). I wanted to add Street Fighter 2 or 4 as a quick lesson on competitive multiplayer, but a Wii package simply has to be added.
If I'm reccomending to a casual gamer who's played before but only now wants to learn more about it: Super Mario Galaxy, Rock Band 2, and either Fable II or GTAIV... probably the latter.
Hardcore gamer: Ocarina of Time (for many, many reasons), Gears of War 2 (for horde and other Multiplayer options), and LittleBigPlanet (for UCC).