I would definetly recommend an ogilvy pad. they fit very nicely on all horses and provide lots of support for horses with sensitive backs. In my opinion they are a little bit more on the pricy side but are definetly well worth it.
For me the most important thing is that the saddle fits correctly. If this is the case then I don't think the saddle pad matters a lot, but a half pad (pad between saddle and big saddle pad) can make a huge difference for your horse. For me it is really important that the half pad does not change the fit of the saddle. For my horse with a very sensitive back I really like the Ogilvy memory foam half pad. It is super shock absorbing and I have noticed a huge difference since i started using it. Unfortunately it also quite expensive, because you pay for the quality :) Another half pad that I use quite a lot is one with sheep skin on the inside, for my idea this just provides a little extra soft layer between the saddle and the horses back, and all 3 of my horses also go really well in this. Hope this helps :)