First, what are you feeding currently? What kind of hay and how much? What kind of grain/feed and how much? Need lbs for both. Any other supplements? How much turnout does your horse(s) get? Good pasture or "not much" grazing?
For the best results, your horse's whole diet and living conditions should be evaluated.
My horses are on 24/7 turnout (just in to eat their "feed") and get free choice (or nearly so) grass hay. For "feed" they get alfalfa or timothy hay pellets, 1/2 cup of flax, and my own custom vit/min/amino acid supplement. I have mine blended for me from Uckele Equine Nutrition. It's suprisingly affordable ($0.49 a day per-horse for mine) and I save a LOT of money over feeding "feed" or fortified grains. The flax adds shine and health to their skin, coat, and hooves, while the vit/min supplement helps with hair/foot growth and overall health.
If you feed a diet similar to mine, but you want an "off the shelf" product, I highly recommend GrandVite. It gave me the best results before going to my own mix. A close second was SmartPak's SmartVite Performance Grass supplement.