My 20-year-old gelding has ringbone and is starting to get a touch of arthritis in his hocks as well, so he currently gets is Acti-Flex Senior, a powdered supplement made by Cox Vet Labs:
Each one has some different ingredients, but here's the salient points: Two different vets (my regular vet and a specialist) have told me that the important ingredients are glucosamine, chondroitin, MSM, and HA. Flex-Max contains 10,000mg glucosamine, 1,200mg chondroitin, 5,000mg MSM, and 150mg HA. Acti-Flex Senior contains 8,000mg glucosamine, 4,000mg chondroitin, 3,000mg MSM, and 125mg HA. (Basically, other than chondroitin the Flex-Max beats what my horse is on now at least in terms of those four important ingredients.)
The Acti-Flex Senior has probiotics that are supposed to be helpful for older horses' digestive systems. However, the Flex-Max has 10,000mg glucosamine, and some people have told me that unless a product has at least that much glucosamine it does not have an effect.
I have yet to compare costs, but I'm considering switching. What do you guys think?