Wii - I have spoken to several people who own the wii (even one avid Nintendo fanboy) and most agree that the wii system gets old rather quickly. Most of the good games for it have already been released and nothing special seems to be in the pipeline.
360 - has come down in price and no longer suffers from the red ring of death problem anymore. Wide catalogue of games and critically acclaimed online system also make it a good purchase. Suit all ages really, from Viva Pinata to Gears of War.
PS3 - is a decent option too. Still a little bit more expensive than the 360, but certainly has plenty of features. Free online service is a nice feature, as is the ability to play Blue-Ray movies (if you have a HD TV). Sadly newer models ditched the backwards compatibility with PS2 games.
Pretty much anything but the wii seems like a decent purchase. I prefer the PC platform more, and for the $7oo+ you will be spending on the console (don't forget extra controllers cost $$$ and games cost a lot too) you could buy or build a nice, powerful computer.
tl;dr : Novelty of wii wears of fast, 360 and PS3 are both good. PC is still cheaper, more powerful and more customizable.