Well, first you need to start with hay and feed. I like mine on free choice grass hay/pasture and ADM's StaySTRONG Mineral Pellets (1-2 lbs a day only). These two things combined gives my horses all of the vit/min/protein they need for their base nutrition. To that I add Cocosoya oil for coat/hooves and Alfalfa pellets for protein/muscle/energy. I have a couple with weak hooves that I give Source FOCUS HF to. It really helps speed hoof growth.
I like to "piece meal" my horses' diet so I can adjust as needed. All 6 of the horses in my care need something a little different to maintain peak condition and health.
If you want just one product for everything, I would look at Platinum Performance supplements. They're pricey, but you have everything covered. You can feed your horse just grass hay/pasture and a little feed or hay pellets to mix in the supplement. Platinum Performance Equine | PlatinumPerformance
If you already feed your horse a fortified feed that he/she is doing well on, then you can add something like Glanzen 3
for hoof, coat, and attitude. This is a flax based supplement with added Biotin and other vitamins. Horsetech.com will also custom blend a supplement for you with only what you need in it. They do not charge extra for custom blends, and their prices include shipping.