My advice is to really do your research on what your horse needs. Feedxl.com is a useful way to balance out all of their dietary needs. It has been very helpful for me to see where I can cut costs and where I need to add a supplement to make sure my horses are getting a complete nutrition.
I have also found that it is usually much cheaper to buy my supplements in bulk (often from different websites than SmartPak because they were available cheaper) and mix them in small baggies myself. If convenience is your priority though, SmartPak is definitely helpful.
If you're curious, my horses get:
Horse Guard multi-vite
Loose mineral salt
Ex-stress (fed for the B vitamin boost because HG lacked them)
MSM (for joints and connective tissue recovery)
Ground flaxseed (for healthy skin/coat and beneficial omegas)
One mare gets raspberry leaves (same thing as Mare Magic) to help with hormone balance. She also gets ReitHoof by HorseTech for better hoof quality. Another of my horses gets peppermint leaves to help with digestion. My mustangs get Animed Remission to help with metabolic issues (cresty neck, etc).