I usually use Absorbine Ultrashield EX, or Country Vet Equine Mosquito and Fly Spray. Both have .50% pyrethrins (the stuff that repels the flies) which is the highest percentage available in equine fly spray. The higher the percentage, the better it works. The Ultrashield is more available where I live and it has the added sunscreen so I use that the most, but I LOVE Country Vet fly spray. It comes in an aerosol can, it's cheaper than other comparable fly sprays, and it's got the best knockdown power I've ever seen. I'll spray it on my horse and he wont even twitch at a fly for days!
I've spent most of my life riding horses. The rest I've just wasted.