Surpass is a non-steroidal anti-inflammatory just like Bute or Banamine. It is only supposed to be used on one joint at a time otherwise you increase the risk of the same GI adverse reactions that you can see with Bute or Banamine. However, it's good if you just have one area to treat such as a single injury. But if you are treating arthritis in multiple joints (common), then you would be better off to use a systemic anti-inflammatory so that you can treat more than one joint at a time.
DMSO is an anti-inflammatory as well but is carries the risk of ending up internalizing anything it comes into contact with. For example, if you have residue from any other medication on whatever you use to apply DMSO, you end up dosing your horse with that as well. Or yourself! DMSO should always be used while wearing gloves for this reason. DMSO is not your best option at all for advanced arthritis.
Steroids are not the top choice for arthritis in horses. They may be used, but not commonly for arthritis unless you are talking about a joint injection rather than systemic use.
For advanced arthritis, the best treatment protocol tends to be a mixture of different treatments. Daily turnout, mild exercise, oral glucosamine and chondroitin supplements, Adequan IM, Bute as necessary for situations where the discomfort is more pronounced and possibly even joint injections depending on the intended use of the horse.
You should discuss all of these options with your veterinarian.