Almost any martial art you pick is good for weight loss as well as muscle gain, cardiovascular improvement, etc. It all depends on what you like.
If you prefer kicks, then Taekwondo is your sport. If you want a mix of hands and legs, muy thai is good. If you like the elaborate, fancy styles, try Kung Fu. I personally like Taekwondo, but I think Wushu Kung Fu is a beautiful style and I wish I had the time to practice it.