I like Senior Flex by MedVet pharmaceuticals, it has high levels of several of the components thought to help with joint problems, and it's competitively priced. I personally like that it contains 5000 mg of yucca, I've had good results with 2 arthritic horses on supplements that were yucca alone. One of those horses (age 20) got markedly stiffer one time within a week after running out of the yucca supplement. Shortly after putting him back on it, the spring was back in his step.
I like using the resources (and ordering from) Smart Pak Equine. They have a page that compares levels of ingredients and prices of each type of supplement. Here's the link (I hope...): http://www.smartpakequine.com/charts/JointCompare.html
Hope you find something to help your horse!