Which breeds make the best trail horse (do I hear a hornet's nest buzzing?). Well first let me state the obvious, whichever breed YOUR horse is, is the BEST for trail riding (hornets nest quieting down).
Seriously folks, if you are just starting out and thinking about the kind of horse you want, here are some thoughts. First, what kind of riding do you want to do? Is trail riding the only riding you will do, or would you like to get into dressage, jumping, reining, cutting, or working cow horse?
While all breeds can do all disciplines (more on this later), each discipline features breeds that can excel in that discipline. Take jumping, for example; most horses can jump over low jumps. Quarter Horses, Paints, and Thoroughbreds often compete in hunter hack and hunter jumper competitions, and lower level show jumping. But if you expect to compete at the Olympic level you will need a big horse such as a Warmblood (sp?) or large Thoroughbred or Thoroughbred cross. Similarly with reining, cutting and working cow horse; Quarter Horses, Paints, and Appaloosas can excel. And even within each, the breeding is refined so that for example, a top level cutting horse will normally not excel at another discipline such as roping or reining. This doesn't mean you can't have fun with other breeds. My horse is a Tennessee Walker who likes to jump logs on the trail and is learning reining and working cow horse maneuvers. We'll never be able to compete with horses bred for these disciplines but we can have fun trying. So it is with trail riding. If you're going to get into competitive trail riding or endurance riding, you might follow the lead of most competitors with their Arabs and half Arabs (although in 2002 the national top point horse in competitive trail riding was a Tennessee Walker -- see you don't have to follow the crowd).
Beyond discipline is the rider's age. Huh? Yep, age. If you're over 40 (my wife and I started riding when we were well over 50) and did not ride in your younger years, I strongly recommend a soft gaited breed (usually just referred to as "gaited") such as Paso Fino, Tennessee Walker, Racking Horse, Spotted Saddle Horse, Missouri Foxtrotter, Peruvian Paso, Kentucky Mountain, among others. Why? Because they don't trot! They all have some form of four beat gait such as running walk, Corto, or foxtrot which does not bounce you up and down like a trot so you don't have to "post". Not that there is anything wrong with posting but if you didn't learn it when you were young.... So for trail riding, think smooth, and not just if you're over 40. Many of our young riding friends ride gaited horses. For younger riders it's a choice, for older folks it's a necessity. And fellas (wives, read this also), I have friends whose wives, who had ridden all their lives, finally talked them into getting a horse to go trail riding with them. Unfortunately, they choose horses just like they had always ridden -Arabs, Quarter Horses, etc. Beautiful horses all, but the guys? They couldn't learn to post (or not physically able) and so they can't enjoy the ride because at some point their friends will want to go faster than a walk. And if you want to do a little cow punching, Foxtrotters were bred to work cattle in Missouri and Paso Finos are used on the ranch in Columbia. And my Tennessee Walkers? Well they were bred to be an all around farm horse with a smooth ride for the plantation owner.
Of course all of this opinion is stated as fact! Because that's the way it's done around the horse riders' campfire.