What a loaded question! I think each system hosts its own perks over the others.
Wii definitely caters to the younger market, the old school nostalgic gamers, the local multi-player party gaming crowd, and the casual types. What it lacks in graphics it makes up for in gameplay.
The Xbox 360 and PS3 would almost be a toss up. The Xbox 360 wins in the online gaming department, while the PS3 shines as a great multimedia device (blu-ray, etc) that won't break down on you. Both systems aim at the moderate to hardcore gamers and their specs essentially produce a comparable high end graphical experience.
With all 3 systems now producing motion sensing games and a plethora of streaming services like Netflix, it probably just boils down to the games you want to play. Which system's library holds more titles that you desire?
Personally, I own a PS3. But I must admit, I spend more time watching Netflix and Hulu on it and leave the gaming to my PC ;)