RepelX (by farnam products) is my favorite too. I use the refill also, and just refill the bottle that I buy in the beginning of the season. 2 parts solution, 6 parts water. Yeah, I use a bit much for solution on the refill, but it does the trick.
Also, adding garlic to your horses diet is a natural way to relieve flies. The "stink" keeps them away.
I use good ol' OFF with deet for myself :P