I love the Equine Senior as well. As he continues to age, you might also want to consider adding beet pulp, dampened high qality pelleted feed or dampened alfalfa cubes.
I kept a beloved old large pony with only three molars left in his mouth in good weight and healthy until 35 by feeding a combination of the Senior and the above.
I'm sorry, I just re-read the original post and realized you're asking about supplements, not feeds. No one supplement will be perfect for every horse on your place. Supplements that I have used with good results on stiff or arthritic horses include: Cosequin, Synovi-G and MSM with chondrotin.