Well, the ultimate multiplayer Wii game at the moment has to be "New Super Mario Bros. Wii". It's a 1-4 player side-scrolling platform game based on the original Mario games, and it's a complete riot to play, especially with 4 players.
Other games to consider are:
Wii Sports Resort - The sequel to Wii Sports. It has 12 new sports and comes bundled with the Wii Motion+ for greater control. The downside is that some games require you to buy extra units of the Motion+ if you want to play multiplayer (but it's worth it).
Mario Kart Wii - Kart-style driving game, and a hoot to play. Allows 4 players via split screen, or 12 players online (2 players per Wii console). It comes with the Wii Wheel, but you don't need it to play, as the game can be controlled with the Wii Remote, Wii Remote and Nunchuk, the Classic Controller, or a GameCube controller.
Super Smash Bros. Brawl - This is a fighting game set in closed-off arenas (though there is also a side-scrolling adventure mode). Up to 4 players can play, and it also includes online multiplayer. Great fun.
Wii Play - Party game. Not the best but a laugh in multiplayer, plus it comes with a free Wii Remote.
Boom Blox: Bash Party - Multiplayer puzzle game. Very competitive and addictive.
Mario Party 8 - 1-4 player party game featuring Nintendo characters such as Mario.