I don’t have any default supps, but the ones most likely to be used are grass balancer (a vitamin/mineral supp for easy keepers), electrolytes and MSM. Of that group, I have had very few horses NOT on MSM. If it were more expensive than it is (dirt cheap), that could change. 
A bit of salt in the summer to keep them drinking.
Adding an Omega3 to my mare’s diet has been a big help since she’s been having a harder time with the bugs down here. Since I started giving it to her, she’s had almost 100% improvement when it comes to being itchy and swollen. She also looks great, maintains a good weight, and has a wonderful coat, even though now she’s on pretty sparse pasture. I’ve noticed her feet are much less chippy as well.
My horses get a supplement with digestive enzymes, probiotics, as well as glucosamine and chondroitin. I also add extra glucosamine. At first I was just giving it to one horse since she was a harder keeper and the enzymes/probiotics really seemed to make a difference, but I started my other mare on it too and her coat got nicer and she just looks great overall. I’ll probably keep giving it to all of them just because it seems to make a difference in their overall health/shine/whatever. My horses have never looked better and my hard keeper keeps her weight better. Also, with my horses that I use hard, they always stay on a joint supplement.
Diamond V XP.
Others I’m fond of, but don’t use regularly:
Calf-manna
Flax
Red Cell
Flax & MSM
Both of mine (one is 21 the other 6) get SmartOmega and TLC from smartpak. My older guy gets farrier’s formula because his feet fell apart when I took him off. The young guy gets TLC which is an anti-inflammatory since when I took him off of it, my trainer complained that he was stiff. Those are my basic supplements and I have noticed positive benefits while they are on them.
I only have one singular horse, but use (and would use on any other horse(s) I will have) MSM and Source. Now that my guy is a Sr., I use Focus Sr., which is Source with antioxidants, pre- and probiotics, flax, etc. He was looking a bit scrawny this spring and the Focus made a huge difference.
The only thing that all of mine get is a scoop of electrolytes during the hot Southern summer. The rest are tailored to each horse, but I don’t give a lot in general. They will all be getting flaxseed as soon as it comes in at the feed store to give an extra shot of fat going into winter
Tri Amino from Uckele and Vit E when there is no appreciable grass.
3 out of 4 are also on MSM.
Mega-Cell (multivitamin) and salt. The two older ones (14 and 16) are on MSM.
Micronised Linseed.
Both of mine get HorseTech’s High Point Grass vitamin/mineral supplement and MSM (also from HorseTech).
I chose the High Point for added copper and zinc (without any added iron) and the selenium. It also has a prebiotic/probiotic. I’m quite pleased with the results so far, and my mare even has dapples for the first time!
My gelding gets loose salt added to his daily snack in cooler weather because he tends to not drink enough.
Flax and msm. Both are cheap so I figure if I’m wasting my money at least I’m not wasting much of it!
