WII GAMES
Well, you have all sorts of party games of Wii, and apparently Wario Ware, Carnival Games, Rayman Raving Rabbids and EA Playground are decent. I believe they all support four players. Also, the Aussies seem to like Mario and Sonic at the Olympic Games.
Other multiplayer games - Ghost Squad is a fun and frantic arcade shooter with 4-player support. The more strategic Resident Evil: Umbrella Chronicles is only 2-player, but I'm sure you could keep switching.
Guitar Hero III is apparently quite good, but you'd have to buy two packages since they aren't selling separate guitars yet.
For sports games, I'd have to recommend Mario Strikers Charged. If you've played the Cube version, it's basically the same but improved and online. It's a great game to play multiplayer. And for realistic sports games, Madden 07 still seems to be the best of the bunch.
For co-op games, Marvel Ultimate Alliance and Metal Slug Anthology have gotten good impressions from forumers.
Probably the only racing game that's worth it at this point is Excite Truck. It is worth it, though..
And finally, if you're a fan of fighting games, Mortal Kombat Armageddon and Dragon Ball Z: BT are the way to go.
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GAMECUBE GAMES
The Cube is NOT compatible with Wii Classic Controllers. If you want to experience Cube games, you have to do so with Cube controllers. But there's tonnes of great multiplayer GameCube games, so I'd recommend looking for cheap controllers. Of course you know about SSBM, but here are some others.
F-Zero GX - My favourite game last generation and hands down the best racing game ever. It's one of only a handful of games last gen that deserves a pure 10. I can't recommend it enough... unless you don't like twitch and reaction-based racing games. It's so fast your head will be spinning afterwards.
Soul Calibur II is probably the best 3D fighter on the Cube. Not much to say about it though.
TimeSplitters 3 (or even 2) are incredible shooting games - I've spent a lot of time on their extensive multiplayer options.
Burnout 2. Hard to find, and if you've played the later versions on other consoles, probably not necessary. Still a great game though.
If you want one of the Cube Mario Party titles, 5 or 7 are the best ones. Mario Golf, Tennis, and Baseball are all okay and worth a one-time rental.
And as much criticism as it gets, I like Mario Kart: Double Dash better than 64. The last few tracks are brilliant.
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VIRTUAL CONSOLE
Not much here, because 4-player is from the 3D era, and there aren't that many N64 games. But...
Gunstar Heroes - this is a great coop 2D shooter. Buy it!
N64 games - the usual (F-Zero X, Mario Kart 64, etc)