i'm using Shoo Fly and it seems quite good , i buy it from ebay, i can't really give you much of a comparison as the only other one i tried was a Naff Off one and it caused a skin reaction
Garlic Granules are supposed to work.. I found Marmite worked (teaspoon in each feed) but horse stunk of Marmite when he sweated - I had to stop giving it!
Science Supplements are bringing out a really good new one. I have had a chance to try it and it really does work! in the mean time Net Tex one works as well.
Flypor. One application along back every month deters all biting insects. It's a chemical base, used on cattle to control lice and flies,but works great on horses too. Other than that, over the counter power phaser is the best, ime