The GameCube because it was the first Nintendo Console that I bought with my own money. The Gameboy/Color/GBA, the SNES, the N64 all belonged to my brothers, so when I finally got something of my own, that made me so happy.
I like the Wii the best. I always rate a system by the games that are on it, and the Wii has the Virtual Console and it can play Gamecube games. Most of the games I like on older systems are on the Virtual Console. That said, I don't actually have a Wii. I own an N64, a Gamecube, and the handhelds from the original Game Boy to the DS Lite. I do play my friend's Wii a lot though.