What is important to you?
Nintendo has excellent first party offerings, but you have to wait some amount of time between quality titles. But you can almost guarantee that the qualities of the titles you waited for will be vast. Pikmin 2, Paper Mario 2 and the coming Metroid Prime 2 are all reasons to keep your Gamecube if you haven't played those. Zelda is coming down the line, as is Fire Emblem (GC).
PS2 is the place to go for incredible third party support. There's niche titles around every bend, and unlike GC... you won't have to wait months between quality titles. We got great games damn near every day this Fall. If third party support is what you desire, there's nothing that competes with PS2. Disgaea, Phantom Brave, Shin Megami Tensei, Ace Combat 5, Katamari Damacy, MGS3, Devil May Cry 3 and GTA:SA are only the tip of the iceberg of the quality titles that have been released or are coming down the pipeline. And let's not forget that Sony's first/second party developers are improving with every passing game - God of War, Gran Turismo 4, Jak III/Ratchet and Clank III, Sly Cooper 2, ICO, Wanda and Collossus(sp) and many more coming down the pipe line top off what is already an excellent PS2 third party game collection. You have much to anticipate.
An Xbox is the choice for your console online gaming. It also has the best possibilities for moding, as is accurately explained in this thread. If you're into moding things and have some extra money to spend to do so, you can't really go wrong. Additionally, while the AAA titles aren't even near as vast as what you can get on other systems, there still are many to like. Halo 2 is just the most recent. There's Full Spectrum Warrior, KOTOR 2, Chronicles of Riddick, Ninja Gaiden, Panzer Dragoon Orta, Crimson Skies, Jade Empire, Fable and many more worth anticipating down the road. It's also the most powerful system hardware wise, so if you're a graphics whore you can't go wrong.
This is my opinion! That is all