Personally I like Tribute Feeds. I use Tribute Essential K. It's a high protein ration balancer. Feed is weighed as per amounts on the feed bag to be fed per horse. For example, my mare is very hard keeper. She gets 4.5lbs of grain split between 2 feedings. This amount is for an 1100lb horse in intense work. She is not quite 1100lbs and works 4-5 days a week for 30-60 minutes. She receives this amount because she is a hard keeper not necessarily her workload.
I supplement with 100% herring fish oil (Golden Horseshoe) for extra Omega 3 & 6 fatty acids. 30cc daily. Some feed companies offer grain with high amounts of Omega fatty acids and this is not needed however, they are not sold where I am located.
I use beet pulp as a filler (soaked)
My hard keeper in full time work recieves 4.5lbs ration balancer (as per feeding amounts on the bag), 4lbs beet pulp (after soaked) and 30cc fish oil.
*Ration balancers give more nutrients in a lesser amount. For example, many other grains/complete feeds would call for double the amount of grain to be fed. While ration balancers generally cost more per bag, you spend more when you are feeding double the amount of other grains/complete feeds.