I had success getting my cat to a healthy weight on perfect weight non-prescription food from Hill's. My dog is currently doing similar on another weight control non-prescription diet.
The prescription foods work as well, but frankly it's more important to actively control food intake than just switching to a lower calorie food imo. Free feeding simply isn't successful for animals who've already demonstrated that they can't control themselves with that kind of feeding schedule. It's definitely a lot harder with multiple pets to feed - when my cat lived with other cats, it was most successful to feed them completely separately and keep the grazer's food either away from where the fat cat could access it or feed them an amount they'd actually eat, but set them into another room to eat more frequently so they could still get the amount they needed each day.
Pets can lose weight on any food so long as calories are controlled (and the amount fed has enough protein for their needs, although they'd still lose weight, just not healthily).