Thineline or Mattes preferably, but, in terms of thickness, Fleeceworks is a nice in between. I've been dying to try one from Padded Ponies though. Basically, if it's adjustable, I'm happy.
I have an Ogilvy, a thin line, multiple fleeceworks, and a gel half pad. My favorites are definitely the gel and the Ogilvy. My Prix de Nations horse had a terrible injury to his back and the only pad that has made him comfortable is the Ogilvy. I also tend to combine it with the gel pad for him just to offer some additional cushion and absorption. We have had many horses come through our program that have had sore backs from years of jumping at a high level and Ogilvy has offered them much relief. I was very skeptical of them at first but I can honestly say they are worth the price.
I like gel half pad, it can perfectly getting adapted to the horse's back :)
Ogilvy's are perfect. But I also like the CSO ones and Mattes.
To me Ogilvy sell the best half pad and saddle pad. They worth their price. My horse has a very sensitive back and it is hard to find a saddle that fits him. My Ogilvy half pad protects his back and does really well its job.