If they are shod, well basically you will need protein, specifically keratin. So a high protein food, amino acids like Lysine. If they are barefoot, other than what mentioned above, a lot of wearing so walking or trotting on cement or in the sand. The movement and abrasion on the hoof will stimulate growth and strength.
My horse lives H24 outside with different types of ground which help him keep his bare feet naturally strong and healthy + has unlimited hay and grass with trace elements, mineral & vitamins as supplements. And for the products i use "Hoof SPA" by a Equivida. It's a natural gel that nourishes the foot, helps growing, protecting & strengthening. :)
Start with the food. Honestly, topical products don't tend to do much-a strong foot starts at it's growth, which is built by nutrition. If there's a lot of cracking going on as well, it could be your farrier leaving or causing leverage on the foot. Do the research, for your horse's sake. :)