The only consensus must-have games are Kinect Sports and Dance Central. Some people like Joy Ride, but there's a demo of it so you can try it out beforehand. I thought it was fun but wasn't particularly responsive enough to justify a purchase. If it were more like $25, I'd definitely get it for a party-type game, but $50 is asking too much.
If you're looking for a fitness game, the only two you should consider are Your Shape Fitness and Biggest Loser. Your Shape sounds like the best all-around fitness game but while Biggest Loser apparently has some horrible menus, I've read a lot of places where people feel like it gives the absolute best trainer-like workout (once you get past the menus).
Other than those (and Kinectimals for the kids), the rest of the games don't really seem up to par, ranging from a "rent or borrow first" to "complete shit".