For starters, it
is a two-horse race between GCN and Xbox. No question about it. Personally, I'd go with the 'Cube.
The Xbox has Halo (dynamite in multiplayer) among others but the GCN has just a
slew of 4-player games; currently available and forthcoming. SSBM is by no means the only one, just the first.. and a taste of what was to come.
PS2 has some great multiplayer games, but with the requirement of a Multi-Tap for 4-player action.. 3+-player multiplayer simply never got the same attention that Xbox and GCN gave it with their built in quad-player support.
Nintendo themselves are big advocates of "friends & family" gaming in the living room, banking on it instead of moving forward with online multiplayer. Many of their own multiplayer-capable games are specifically designed for easy 4-player gameplay.. and the same holds true for close 3rd-party friends like Sega's Amusement Vision and Hudsonsoft. (who developed Mario Party 4/5 and Bomberman Generation) In fact, the GameCube may hold easily the biggest multiplayer-focused game of the holiday season with the
16-player capable
Mario Kart: Double Dash!!..
I know there's a lot coming for my Xbox too, but I think I'll just go on talking about the GameCube.
Upcoming titles include Final Fantasy: Crystal Chronicles. How about Pac-Man Vs*? Zelda: Four Swords? Quite possibly the definitive living-room multiplayer game, Mario Party 5? Plus, Iwata's concentration on connectivity is largely focused in the living room with multiple players. I just don't see how anybody can downplay Nintendo in the area of multiplayer.
Yes, they f'ed up on the N64. Yes, the GBA needed an internal light 2 years before it officially got one.. but damnit, their focus on quality multiplayer has been there! Sheesh, this just went from an opinion into a full-on rant..
* Free