Yup, what aabqh said. Oral supplements carry widely in how they work for different horses. If something works for your horse, you have to stick with it.
Injectable Hyaluronic Acid (sp?) has been shown to have good results as well. The generic form isn't even that expensive. I used it with my 90 lbs Doberman and saw very good results. We was like a puppy again!
As far as oral joint supplements go there isn't a right or best answer. Opinions vary widely from owners and research hasn't narrowed down exactly what works the best and to what degree, it is an ongoing process. There are a lot of articles out there on the topic but no definitive answer. The main ingredients in joint supplements are Chondroitin and Glucosamine. HA and msn are also added to many though in certain states msn is not allowed for regulatory reason in joint supplements and must be fed seperate. The main thing to remember is to buy from a reputable company where the gauranteed analysis is not in question and different problems will respond differently.
The more expensive approach for joint health is Adequan which is an FDA approved drug administered IM, not a supplement that does have research to suppurt claims of reducing inflamation in joints. Will it help your horse and is it cost effective? That's up to you and should be discussed w/ your vet. There are a lot of liquid HA supplements out there also that some people love but the price is nearly as high as Adequan on some.
Another thing to consider is that research has shown that Omega 3 fatty acids help w/ reducing inflammation. The flax seed is fine especially when conserving money but a marine based omega 3 like EO3 oil is absorbed better due to the chemical properties and is easier to digest. We all have to know when to say when when buying supplements and adding thing to the horses diet.
That is why I use dac Oil for omega 3.....it has fish oil included. Before I sold him, my barrel horse was on Herbal Respond for his ouchy days and his joint supplement was Optimum Flex. The links will give a lot of details about what is in them and how they work.