PS2 > GCN > XBOX
Its true that the GC lacks 3rd party support but they have a lot of 1st party games that'll stay in the gamecube for months.
PS2 has a lot of cool games but most of their games are bullshit. A lot of PS2's 3rd party games are random, meaning that they will never be a good game, they're just releasing it to brag that they have more games than the GCN and Xbox. Like last night, I went to best buy and browsed through the PS2 aisle and I realized that only a few of the PS2 games were worth buying. Not so with the GCN. When I browsed through Nintendo's games, even though there weren't as many games as the PS2, they had a lot of games worth buying. The ratio of quality games between the GCN and the PS2 is 2:1 IMO.
XBOX, I can only note a few games that I really want and thats Rainbow Six 3. It seems like Microsoft isn't showing much effort to boost XBOX's sales because you know why? They don't need to. They never needed an XBOX, they're a monopoly in the PC martket, why even bother going for the console market? They were ambitious, thats why.
Cliff Note: Quality > Quantity
Games considered worth buying:
PS2: GT4, GTA (bonus pack), Final fantasy X-2, FF XII, MGS3, Socom 2, THUG, SSX3, Time Crisis 3, NFSU
GCN: Mario Kart Double Dash, Mario party 5, new zelda, new mario, starwars rebel strike, F-Zero GX, NFSU
XBOX: Rainbow Six 3, halo 2...