Pretty much everything I've read has suggested that wool pads are best for dispersing pressure and keeping the horse's back cool (or as cool as possible), which is a bit counter-intuitive, but I guess it breathes better than some of the synthetic materials.
100% wool has been suggested, but that can get a bit pricey, so I think wool blends will work okay also.
Wool also shouldn't slip too badly, or at all.
Whatever you do, avoid the pads with gel in them. They're very heavy and don't seem like they'd allow for my breathing. In addition, I've read a number of negative reviews on them, saying that they actually create more pressure points, as the gel pools in certain areas.
It makes sense, logically, to me, that wool would offer the best weight dispersal and shock absorption, as it doesn't seem like wool that thick could compress too much.
5 stars seems to have nothing but rave reviews, but are a bit pricey for me.
I have found some other similar pads, haven't decided which I'm going to go with, just yet -- Classic Equine Wool Felt Pad. Smith Brothers 100% Wool Pad - Smith Brothers Saddles Tack Horse Supplies - ChickSaddlery.com Mustang Wool Contour Wool Pad
The wool pads always come in neutral colors, but then you put any pretty saddle blanket, that you'd like, over the wool pad.
Another option would be, getting one of these wool pad protectors -- Saddles Tack Horse Supplies - ChickSaddlery.com Wool Pad Protector
and put it under whatever saddle pad you want to use. That way you still get some of the benefits of having the wool directly on the horse's back (although you won't get the same kind of weight distribution as you would with a thicker wool pad).
I'm probably going to grab one of those wool pad protectors and one of the wool pads I listed above (after trying to see if I can find some reviews on them), in the 3/4 inch thickness. Between those two wool pads and the handful of decorative Mayatex blankets that I have, I should have pretty decent options as far as thickness and look go :)
If your saddle fits super well, I don't think you have to be QUITE as picky with the saddle pad. In my situation, my saddle has to go on a variety of horses, so I need the best pads I can afford.