I had a tough choice with this generation of games. I still love playing Atari 2600 (for me, very few things beat Pong), of course my PC, NES, N64, and my PS2 (I think this is one of the best video game platforms in our lifetime to date).
I have a Wii that I received as a gift. I really enjoy it but I am not a fan of most games for it. Some games excel with its unique play format. Wii Sports, Mario Kart, and...damn the Operation Game that I play at Steve D's that I can never remember the name of. They work very well with it. Other games just port into Wii but do not maximize (or incorrectly maximize) the Wii playability features.
I have a hard time adapting to shooting games on the Wii and games that require use of the "Classic Controller." I'd rather play those on PS2 or PC but the Wii does have some major strengths.
As an aside, I think XBox Live is a waste requiring added expenses (like a wi-fi converter/signal box thing really needs to be that much...just allow it to connect to one's home network wireless with integrated wi-fi, which MSFT could do with a nice deal to themselves or Linksys or something).