What has worked best for my boys is Pyranha Wipe 'N Spray. Middle-of-the-road price wise for my area, and it does a good job for a decent amount of time. I recently tried some DuMor fly spray from TSC -- only $5 US, had to give it a try. It's pretty effective for the money.
I found a wipe-on fly repellent gel/sunblock at a salvage freight place that I love. A little blob over the muzzle and ears keeps all but the toughest bugs off of what the fly mask doesn't cover, and keeps their noses from getting sunburned. I may cry when I run out -- the place I got it hasn't gotten any more in, and I can't find the brand or even a similar kind of product even online.
Bronco might as well be overpriced water for all the good it does for my fellas. I can shoot a fly point-blank with the stuff, and it won't even notice. Gordon's gave both of mine giant hives.
Rio is very thin-skinned; even with the best stuff the flies bother him quite a bit. No brand I've tried works for longer than a couple of weeks; I rotate a couple of different brands/active ingredients with homemade recipes to keep the bugs from building up immunity.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown