At the moment though, the market would say Mario Kart, Super Smash Bros. Brawl and Guitar Hero 3 have got some of the best mltiplayer/co-op and online setups going right now.
For really good (not online) multiplayer (besides the above and Wii Sports) I really like The Simpsons, Mario and Sonic at the Olympic Games and LEGO Indiana Jones.
Mario Galaxy and Zack and Wiki have got a multiplayer mode that lets the second player "help" in different ways. Check out the metacritic link below and click on the game for more details.
If you are looking for other (not necessarily multiplayer) really good Wii games, I own 9 of the top 10 rated Wii games from Metacritic.com (see link below). I wouldn't rate them in that order, but if your looking for good games you could do worse by not checking out this site.
Hope this helps!