What more can be said about Wii Sports that hasn't already been said? Despite being a tech demo it has cemented itself as one of the best selling games of all time (even if it was a pack-in game with the Wii). It's really simple pick up and play fun that anyone can play.
It might be really really good, but I personally prefer Wii Sports Resort, since that is a much fuller game with a lot more diverse content.