I hope you also accept Xbox 360 answer, not just the 'old' Xbox :).
On our Xbox 360 we play loads of Just Dance lately - it is a great game that during the game also helps you to stay fit and lose a little bit of weight.
Eversince I've tired it I've recommended it to many friends.
Our other old favourite is the Scene it!-series with the special controllers and movie-related gameplay (I wrote a review about this - search within my hubs) - in the past few years we almost always used these games on parties in our home as it seems to be everyone's favourite since the first time.
I can warmly recommend both!