I'd have to say that it depends on the type of gamer that you are.
XBOX360 caters to people who enjoy playing online with other players, it's the main focus of the console. Sure the service is pay-per-month, but it's easy to communicate with your friends, organize groups, and play together. Keep in mind that the hardware is flawed, but warranty covers that.
PS3 caters to people who enjoy media. The Blu-Ray factor alone sells people on the system, and now they're coming packaged with HDMI cables. You should go for the PS3 if you want to game, but at the same time want something thats good at movies, music, has a decent web browser, etc. Another selling point is that its hardware fails a lot less than its main competitor.
Wii trades off raw power for novelty. While you often can find a wii version of a game thats also on XBOX360/PS3, its more than likely going to be a dumbed down version to fit with the lack of processing power, and to also fit in the novelty of the use of motion controllers. The exclusives are where the best games for this system are.
Just ask yourself how you want your gaming experience to be, and choose the system that fits your needs the best if you're only planning on owning one of the consoles.