Mario Galaxy I and II
>> New Super Mario Bros Wii (I watch my 6-year-old play this and frankly marvel at his skill – it allows for incredible proficiency if you try)
>> Mario Kart Wii (the rubberbanding will drive you crazy, but it is brilliant)
>> The Legend of Zelda: Twilight Princess
>> Metroid Prime Trilogy (more adult; not for kids)
>> Metroid Prime: Other M
>> Goldeneye
>> Super Monkey Ball (my boys love this game)
>> Wii Sports (don’t discount this excellent set of mini games)
>> Wii Sports Resort (I think this is possibly reason enough to buy a Wii and the attendant motionplus controllers in and of itself – BRILLIANT fun)
All these games aside, I think the greatest thing about the Wii is what you can do with it when you jailbreak it:
> SNES Emulation
> Genesis Emulation
> PS1 Emulation
> Gamecube Emulation
> Multiple other awesome things