The N64, because it's what I grew up with. I do however, love the NES just as much because the games were harder and much more challenging in my opinion. The SNES is a good system, but its library was mainly RPGs, and while I like RPG games, too many can be a bad thing. I enjoyed my time while playing the Gamecube, and many of the games were great, despite most of them being sequels/prequels/other -quels, and not many original/exclusive games on the system. The Wii, however, is my least favorite system out of the list. With all the shovelware out there, and the fact that most games don't use the "innovative" controller, it's hard to find a game worth playing. I haven't bought a new Wii game in about a year, and I just wish more games used the Wii remote/nunchuk in a creative way.
Also, I own all of these (Except for Gamecube, which I owned before the Wii), so my opinions are based off of my experiences with them.