IMHO, average horses do best on a diet of good quality hay. In some areas, horses need some ration balancer or additive to make up for some nutrient or mineral that is lacking in their available diet (I think selenium is one of the most common needed). Other than that, horses don't need anything special by way of feed.
I have anywhere from 13-25 head being fed at my place at any given time, has been that way for decades. Since we stopped feeding sweet feeds in the early 90's and went to an all hay diet, we've not had a single instance of colic or founder or any of the other food-related illnesses so common in horses. Maybe it's just because I'm lucky, but I figure that their diet plays a very large part in it.