I really rate the GWF supplements, I've used their joint one on all my horses for years and really rate it. I think which you need will probably depend on what you're looking for it to do - prevention/ maintain or help once joint problems have started.
Boswellia, MSM & chondroitin, and green-lipped mussel are all evidence-based supplements that support joint health and/or reduce inflammation, though the bioavailability of GLM in horses is a bit questionable as they're not really equipped for digesting animal products.
I buy powdered boswellia in bulk from Redpost equestrian.
I've been feeding Equicure flex. Two key things I was looking for were green lipped mussel and Boswellia and it has both as well as other thiny. I was feeding yumove or maxaflex but vet suggested adding Boswellia
Boswellia is good stuff - not quite strong enough vs bute for mine but got him about 75% there. It also made him super dopey and spaced out so beware (mine's weird though). I thought I saw a difference with GLM but when I removed it from him I didn't really see a difference so stopped. MSM does seem to help though and is cheap as chips.