Super Smash Bros. Brawl
Metroid Prime Trilogy
Mario Kart Wii
Resident Evil 4: Wii Edition

I'd recommend RRR TV Party. Girls love it *hint hint*.

1. Super Mario Galaxy ( the game is awesome ).
2. The Legend of Zelda - Twilight Princess - ( this game is a ZELDA game ).
3. New Super Bros Wii ( Here We Go !).
4. Metroid Trilogy ( if you like the genre you can't go wrong with it).
5. Resident Evil 4 Wii ( the best port game ever).
6. Okami ( ART ).
7. Super Smash Bros Brawl (go check reviews by yourself).
8. Mario Kart Wii ( Easily beat its 20 Millions unit sold !!!).
9. Naruto Clash of Ninja REV III ( one of the best fighting game based on an anime )
10. Wii Sports Resorts + Wii Motion plus ( discover how great is the WM+ ).

It really depends on what kind of games you are looking for

Obviously there are the top platfomers - Super Mario Galaxy, New Super Mario Bros Wii - Both of which are incredibly fun, but I'd pick NSMBWii over galaxy if you are just looking for one, but then again I'm 2D biased.

For Light Gun shooters, there is Resident Evil (Umbrella chronicles and Darkside chronicles) there is also Dead Space Extraction. The resident evil games are supposed to be better, but I haven't played them, but they are worth looking into.

Trauma Center: New Blood can be found for $20, and is a great game. You play as a surgeon and heal people as fast as possible from ridiculous situations and diseases. It is also co-op, which makes it even more fun.

Geometry Wars: Galaxies - If you like shoot em up type games, you fly in a space craft and shoot lots of bad guys who have various behaviors. The entire point is just to get high scores on various stage designs. It is quite challenging. I'm sure you can pick it up in the bargain bin for $20 or less.

Super Smash Bros Brawl - not much explaining needed. Great game if you like the series. If you're not familiar with it, 4-player fighter chaos pretty much explains everything.

Okami - if you are an action/adventure fan. The story can drag on, and but the game is good. If you like Zelda there is a chance you'll like Okami as they are very similar.

Mario Kart Wii - best kart racer around. The online is superb and a lot of fun.

ok, my list is getting cut short by my crying kid, but I'll also throw out...

Punch-out, tiger woods with WM+, Wii Sports Resort, Muramasa, Little King's Story, and more...

NSMB Wii -- this will keep you tied over for a while
Silent Hill: Shattered Memories -- great experience
Wii Sports Resort
Muramasa: The Demon Blade
Little King's Story
House of the Dead: Overkill -- short-lived (maybe 4-6 hours game time) but hilarious!
Mario Kart Wii -- entertaining, but not as strong as the DS version sadly enough.
No More Heroes -- buy it now if you haven't played it (I think it's 20 USD)
Smash Bros. Brawl
A Boy and his Blob

Substitutions:
Punch-Out!
Madworld
WarioWare: Smooth Moves!
The Conduit

-------------------
For Wiiware, consider the following:

Lost Winds
Lost Winds: Winter of Melodia
Toki Tori
World of Goo
MEGA MAN 9
Strong Bad's Cool Game for Attractive People
Bit. Trip Beat
Bomberman Blast
Swords and Soldiers
Tales of Monkey Island -- some lag though

I think his list is a pretty good one. I would just add for your consideration Rune Factory Frontier.

1. Super Mario Galaxy
2. The Legend of Zelda: Twilight Princess
3. Mario Kart Wii
4. New Super Mario Bros. Wii
5. NO MORE HEROES
6. De Blob
7. Super Smash Bros. Brawl
8. WarioWare: Smooth Moves
9. Muramasa: The Demon Blade
10. Resident Evil 4: Wii Edition

(add Sonic Unleashed, if you like Sonic)

I reccomend gamefly.
10 games is 500$ which could buy 2 game gamefly for 2 years :o

Zelda: TP
Mario Galaxy
NSMBWII
MKwii
Smash Bros Brawl
Silent Hill: SM
Paper Mario
???????????

Holy shit. You know about Goeman? Bonus points in my book. =]