Bomberman '93. Hands down one of the best party games I've every played. It's a riot, it's so easy to pick up and play, you can use the Wiimote AND it has support for five players (using a GameCube controller.)
For some reason the multiplayer on mine has a glitch. Whenever we try playing it, Player 1's controller stops working one the game starts.
We had one Wiimote (player 1) and four GC controls hooked up, if that makes a difference.
I also reccomend Kirby 64. Easy game, lots of cartoony graphics, but a blast to play and it has one of the most epic final boss musics in game history (I'm not kidding, that song is awesome).
Furthermore, any of the R-Type or Gradius games you can find. I'm a huge fan of shmups, so I try to get as many of those series as possible (I am irked that R-Type II is only available in Japan... really irked). But I warn you: these are extremely difficult games (R-Type got ranked 7th on IGN's "hardest video games ever" list). Just getting halfway through is an accomplishment. So it is a great feeling when you manage to beat them.