Wii Sports -- Everyone has this one, and everyone has been praising it since it came out. All I can do is echo the praise you have to give Nintendo for not only coming up with this beauty, but also for packaging it with the Wii.
PBA Bowling -- My wife liked bowling so much that we picked up this game when we got the Wii. It is a harder bowling game, but not really much more fun than the bowling that came with Wii Sports. The graphics are nice though and funny characters.
Guitar Hero III -- I had never played Guitar Hero before this, and I can see why it has been such a huge success. Unlike a lot of Wii games, it is not really a great game to play with friends. That's alright, it is incredibly addictive and fun. I got so into the songs on here, that I had to buy several of them off of iTunes.
Lego Star Wars -- This game is a lot of fun. It's simply the Star Wars saga with Lego graphics. Very simple, but still cool. The best thing is getting to use the Wiimote as a light saber. I told my wife "I've been waiting for a game like this for 30 years!"
Madden 2008 -- I haven't played this much yet, as my wife bought it for me the day we were flying back from Florida. She had wanted to buy it for me for Christmas, but it was very hard to find in Florida. I have only played one game, but the controls rock! There are a lot of things you can do, but they are (surprisingly) very intuitive. I can't wait to play a Franchise or a Superstar.