I've posted a list of my favorite Wii Fitness Games and Xbox Fitness Games here:
Wii Fitness Games:
http://www.nutwiisystem.com/best-wii-exercise-games.html
Xbox Fitness Games:
http://www.xboxfitness.org/best-kinect-fitness-games/
This list came about after a lot of testing.
Bottom line, as far as "pure fitness games" so, it's hard to beat EA Sports Active 2 for Wii or Your Shape: Fitness Evolved for Xbox.
However, if your goal is to just have fun and lose some points along the way, Just Dance 3 for Wii or Kung Fu High Impact for Xbox are my favorites.
Whatever you choose, the key is to play it for a sustained 20-30 minutes at least every other day until you elevate your heart rate. Do that, and you'll definitely feel more fit and as long as you keep your diet in check, you'll lose weight too!