Hands down wait until Super Smash Bros Brawl comes out, March 9th.
With the options to play with random people online, or match up with any Wii Friend (i.e. anyone on this forum) it'll pretty much be the hottest thing available for the next couple months.
Mario Strikers is fun too, although only if you like sports games.
Same with Medal of Honor, but thats only if you like first person shooters.
Difficulty-wise, I don't consider SSBB that difficult to get the hang of. Just the advanced tactics and things will take some practice (if thats your thing)
Other than that, you don't really have a lot of options for multiplayer on the Wii, unless you're into Pokemon Revolution...