A saddle pad that fits right is an essential piece of tack. With the right saddle pad, saddle, and riding posture, you can ride a horse for tens of thousands of miles over the course of the horse's lifetime and never cause a single sore on the horse's back. However, with a poorly fitting saddle pad, saddle, or poor riding posture, you will certainly cause sores along the horse's withers, or spine, often within an hour or less.
For my own part, I have, for the last several years, used only two types of saddle pads. For the horse that is either very small or else is very mutton-withered or overweight, I use a "Casa Zia" saddle pad. These are simple, alpaca-wool pads that are not too thick and help keep a saddle from rolling on the back of such a horse.
For every other horse, that is, about 95 percent of the time, I use a blanket-top felt saddle pad by "Classic Equine". These pads fit well, and are slit down the middle with a hole over the withers to allow for plenty of breathing. The wool blanket-top comes in several colors and designs so they also look very nice. Whenever I sell a horse, or help someone train a horse, especially if they are new to western horsemanship, I advise them to purchase such a saddle pad.