To me, hay is the most important part of the diet. Which is also what most people seem to neglect mentioning the most. People often say "hay." But what kind of hay?
My horses get approx. 50/50 alfalfa/bermuda hay. Actually, I probably lean a little heavier to the alfalfa side. Here in Arizona, those are about the only two hays we can get consistently. A lot of people knock alfalfa, but it is a good hay. No need for extra protein or calcium supplements when you feed alfalfa.
Then with their dinner meal I will give them a "treat" of grain. If I have coupons I might buy them something by Purina such as Strategy. But other times I might just buy some sweet feed. They are fed according to need and energy level. The riding horses get 1-2 pounds of grain and the long yearling gets only about 1/2 pound. Just enough to mix in a couple scoops of vitamin/mineral supplement. They only get their "treats" once a day. The riding horses don't even get the vitamin supplement. Just the yearling because he is growing and I want to make sure he isn't lacking any nutrients.
I do sometimes pour a little vegetable oil over the grain on the riding horses because it gives them a lovely coat.
So I really focus on feeding a good mix of quality hay, and then their grain is more of a treat.