I think it depends on the kind of trails you ride as well as the distance and frequency. Your size, riding ability and preferences are important too.
Soundness is a big requirement though. Good feet, sturdy legs, strong back.
I also want a trail horse that takes new things in stride, not spooky, balky or stubborn. Sure-footed, thinks on his own so the rider doesn't have direct each step.
Do you like to ride slow or move on out? Some breeds naturally walk slow and some walk fast. Some pole along happily and some don't like to be held back.
Does your horse match your weight. Some breeds of horses are bred to carry weight better than others. It isn't just about size, more about how they are built.
Do you like to take out alone or prefer ride with others?
Some horses are bred for strength or endurance and others for speed. I know that QH people go on about how perfect the QH is but they don't win endurance races nor can they pull like a Percheron. Nothing wrong with QH just that they are not bred for endurance or pulling. Let each breed have it's strong points and not try to fit a horse into something he is not bred to do.
So decide what you like to do and find the horse that can easily give it to you.