A wii, by far, it has more kids games, it actually encourages more physical movement and the games are geared more towards children, xbox 360 is more for adults. ps2 wouldnt be bad, but the technology is getting kinda old and besides the wii, has really came down in price. We have PS2, Gameboys, Wii's game cubes, Nintendo, super nintendo and nintendo 54, the wii is my kids hands down favorite to date.
They love the physical interaction and when paired with the wii fit, its a nice program for the whole family. btw the Wii has just dropped their price down by $50 and you will probably see some great bundles on black friday.