I hear Excitebots is loads and loads of fun.

Personally, I'd recommend trying out Punch-Out!! and The Conduit once they get released (Punch-Out!! = May 18th, The Conduit = June 23rd).

MadWorld is pretty fun too, check out some videos and if you think it looks interesting you should probably pick it up.

im Waiting for Punch Out, Pikmin 2 and Little King's Story... Pikmin 1 Wii is very good...

Madworld and Bit. Trip Beat on WiiWare
(>'.')>

Anyone thinking of buying Pikmin Wii should really buy Little King's Story instead. It takes some of pikmins core gameplay mechanics and expands it into something much bigger.

I'm not saying pikmin is bad, so if you like it buy both. But I think LKS is overall a better and more enjoyable game and really needs the sales. If you're a fan of pikmin you really should try it, and if you haven't played pikmin I think LKS is a lot safer bet.

Ruin Factory Frontier.

You can try these games:
-House of the dead overkill
http://www.youtube.com/watch?v=6NVb_EE55AE
-Madworld
http://www.youtube.com/watch?v=rdqD7oTH6EM
-Scarface (if you like GTA)
http://www.youtube.com/watch?v=3zRozwtnWWc
-Manhunt 2 (hoho, if you have courage)
http://www.youtube.com/watch?v=GAIiN_sIBc8
-Pikmin 2 New play control
http://www.youtube.com/watch?v=TeN-Hkg2iyw
-Donkey kong new play control
http://www.youtube.com/watch?v=DYhf70z4S58

Sorry, i cant tell you more, i am playing these games, i am new in the Wii...

@Edit:

I forgot of these games:

-Trauma center (is something totally new, you have to try)

http://www.youtube.com/watch?v=HXbjD4p-hjw

-Fire emblem (a strategy game)

http://www.youtube.com/watch?v=NcavcNewIBM

-Excitebots 

http://www.youtube.com/watch?v=Hht8wyPQVdQ (the quality of this video is a crap)

-Wario land: Shake it (is a pretty fun game)

http://www.youtube.com/watch?v=S49gYrN9zwI

-Rune factory frontier (excelent game, is a mix of RPG + Harvest moon)

http://www.youtube.com/watch?v=JKw8cquUYcU

-"Wiimake" of the Resident evil series (I don't like ports/remakes, but what can I do? these are good games...)

about the Punch Out! : You have to play the snes version, this is a great game and have great characters. I am excited about the wii version.

If you're looking for a newly released game, go for Little King's Story. I haven't finished it yet (in fact I'm not even half way through) but this game is simply amazing. There is so much to explore, I guarantee you will spend hours traveling through the world in search for secrets.

Another must-buy is Okami. Definately one of the greatest games of all time. If you like Zelda-type games you cannot ignore this masterpeace. The graphics are awesome, the story is epic, the music is beautiful, the battles are fun and the gameplay is almost  perfect. It's a shame that it sold so poorly (on both, the PS2 and the Wii).

Super Mario Galaxy has to be mentioned as well. Arguably the game with the best visuals and the best score on Wii this far. And the gameplay itself is awesome, too, thanks to the wonderful and innovative level design.

If you prefer a more action-packed game you have to look for Metroid Prime 3: Corruption. The combination of shooter and adventure elements works perfectly and the accurate aiming shows, that the Wii-mote is by far the best controller for FPS. I would say it is at least as good as the first Metroid Prime on the good ol' Gamecube.