For feed thrus, I've tried Simplify, which works great, but only if all of the horses on the property are on it. My favorites for fly spray are the Ultra Shield EX and the Country Vet Active Equine fly spray. The potency of fly spray is based on the amount of pyrethrins in it, which are the fly repelling chemicals. Both the Ultrashield and the Country Vet have .50 % pyrethrins in them, which is one of the highest amounts you can get. The higher the pyrethrin percentage, the better and longer the flyspray works. Fly sprays like Bronco are cheaper because they're only .06 % pyrethrins, so they work pretty poorly. You're better off spending the money and buying the good stuff with a high pyrethrin count. Just look on the flyspray label, it'll say the pyrethrin count right on the front.
I've spent most of my life riding horses. The rest I've just wasted.