I prefer plain rice bran over products like Amplify or Horseshine if you can find a stabilized rice bran. Unstabilized rice bran goes rancid very quickly, before it even makes it to the feed store shelf in some cases. Depending on the rest of your horse's diet, you might also need to have one that's fortified (added calcium to offset the high phosphorous content). Finding a stabilized, fortified rice bran might be a challenge in your area (I had to look at 3 different feed stores in my area to find it!) and it will add a lot to the cost. At that point, Amplify, Horseshine, etc. can be cost competitive and may be easier to find (and these ones are already stabilized & fortified)
However, I really like flaxseed more than any of those. It's higher fat than rice bran (rice bran is ~20% fat, while flaxseed is >30%) and is much better balanced for omega-3 fatty acids. This is especially important for horses that aren't on fresh grass, as they usually don't have a good source of omega-3's but get a bunch of omega-6's from pelleted feeds & grains.
Triple Crown makes a stabilized ground flaxseed called Omega Max, though you can also feed whole seeds or grind your own (though if you do grind your own, do it fresh daily)