I use Tri-Tec 14, with great success. If you're worried about the cost, you can go to Farnam - Your Partner in Horse and Pet Care™
and print a $2.00 off any size coupon. I recently printed myself five for future use. The great thing about the Tri-Tec 14 is you don't have to spray them daily, so it lasts longer than most fly sprays. To me the cost of it is well worth it, because put simply, it works, and I only have to spray my horses about every 10 days instead of daily. And, with 4 horses that makes it pay for itself.
I also put out Fly Blocks with Rabon to help cut down the fly population, I get them for $11.99, and the first one of the year will last about a week or so, but the rest of them last longer; for some reason the horses always pig out on the first one that I put out.