I believe The Beatles: Rock Band came the closest to being perfect. Eveything about that game was just beautifully done and the only things missing was Pro Drums, an option for Keys, and a few more Beatle songs (Hey Jude, Strawberry Fields Forever, Yesterday etc. etc.)
Similar to the OP, the one I would settle on is Dance Central 2 for Kinect. The music is great, the dancing is great, and the controls are singular (your body is the controller).
For a more conventional choice, I’d say Beyond Good and Evil. The story and world are immersive and novel, and the stealth gameplay is also really fun.