Depends on what you're after, and who's going to use it, such as age of the persons among other things.
Wii is a great party-system or a nice family-fun machine. It's far from a hardcore-gamesystem, it has pretty low graphics for example, but for a family with younger kids it's the right choice. The whole idea is to have fun in group, through moving and useage of your own body.
Xbox360 and PS3 are both really good systems, more hardcore games and better graphics, more power in the machines. The Xbox360 can play DVD movies, and the PS3 can play both DVD and BluRay movies. Both systems can store music. PS3 supports online gaming for free, Xbox360 takes a fee - but their online-mode is a lot better and easier to use. PS3 has more power, and can, if the game-developers really try to get the most of the machine - show better graphics than the Xbox360. Both systems has a lot of good games, both systems has a few game series exclusive to their own system - for example Halo on Xbox360 and Uncharted series on PS3 (to name a few). Both systems now support the same idea that Wii is founded on - gameplay through moving your body. The PS3 version is called "Playstation Move" and uses a hand-held controller, much like the Wii. It usually costs about 100 US dollars (this is a item that needs to be bought outside the system itself, or together with it as a bundle for a reduced price) with a game included. (I checked the price on BestBuy). The Xbox360 version is called "Kinect" and uses NO controller, only the movement of your own body - you are the controller. It usually costs around 150 US dollars (this is a item that needs to be bought outside the system itself, or together with it as a bundle for a reduced price) with a game included. (I checked the price on BestBuy).
So, to sum up:
The Wii is probably the best system if you're a family with younger kids who wants to enjoy some fun, nice, relaxed family time together, this is also the cheapest system.
The Xbox360 or PS3 is a better choice if you want to have fun with your family but still be able to play hardcore games, have movienights and watch DVDs and/or Bluray. These two systems are more expensive, with the PS3 one being the most expensive (but at the same time the system with the most power). Both these systems support movement-play, with the items Kinect (Xbox360) and Move (Playstation), but these items will add another cost.
Hope this helped. :]