Why do you want a prescription veterinary diet food? Why not just feed a good quality food in the appropriate amounts for weight loss? All of the "veterinary" foods are going to be very high in grains, not conducive to weight loss. A good quality, healthy food fed in smaller quantities and coupled with exercise will give you the weight loss you are looking for.
If you really insist on feeding a "vet" diet, then go for the Royal Canin. At least there is a shred of meat in that one. Still not good ingredients though.
If you are willing to stray away from the "vet" foods, there are some good quality "weight loss" or "low fat" foods out there. Innova Reduced Fat, Wellness Core Reduced Fat, for starters.
But really, if you go to your human doctor and you are a little bit over weight, what do you think your doctor is going to recommend? High quality, fresh, natural ingredients in smaller quantities, cut out the junk food, and increasing your exercise regiment. Not some artificial weight loss diet, or fad diet program.