A well-fitting pad starts with a well-fitting saddle. If you start with a well-fitting saddle, it is likely that you will not need an extra thick pad under it. Always use a contoured saddle pad to ensure a smooth fit under the saddle, so that your horse does not feel any 'wrinkles' on his back.
The western pad should help to wick away sweat, distribute heat evenly, and absorb and distribute the weight pressure from you and your saddle.
The best pads to compensate for the saddle's inability to conform to the change in your horse's back are made from natural fibres. Natural fibres are naturally resilient and have a resilient capacity. The natural fibres also have the ability to hold air to help dissipate pressure and have a natural ability to wick away sweat. Because every horse has pressure points, natural fibres have also been shown to distribute weight and pressure better than synthetic materials.
Buying a good western pad is an investment in your horse's back and can ultimately save vet and chiropractor bills.