I like just about any thick (3/4 -1 inch) wool/wool blend pad.
If I had all the money in the world and could buy any saddle pad, this would be the one -- 5 Star Wool Saddle Pad
But I can't justify paying more than like $150, so I go for a cheaper variation of the one linked. Classic Equine makes some pretty good ones.
I look for straight wool/blend, no neoprene or anything like that. Definitely none of that gel business. I like the contour pads and the wither cutout.
I then have some pretty mayatex blankets that I will use over the wool pad, if I want a nicer look. I know people who put a blanket under their wool pad, since you can't really clean the wool pads (other than taking a hard brush to them), so they just wash the blanket. But I like having the wool pad in direct contact with the horse's back, because I feel that wool keeps their back coolest.