Choose something with the highest amount of glucosamine you can find and that also has MSM. In one of the few reputable studies available on joint supplements in horses, this combination was shown to be the most effect.
However, you may want to also consider going with something that has more research behind it and that tends to be more effective than oral supplements. Talk to your vet about using Adequan IM which helps to protect and rebuild joints. And while the initial cost of Adequan IM is fairly high, after the first month you go to once monthly treatments which tend to run about the same or less than supplementing for a month and you will get better results.
Licensed Veterinary Technician