There are several options, and I don't have an opinion on which is best. I think it varies with the situation.
One is the one-rein stop, which is not supposed to be "Pull the horse's nose to your knee and force him to stop". That is how I was taught it, but I've ops checked it on a gelding and we nearly went thru a barbed wire fence. Horses CAN gallop with their heads twisted around.
I believe the one rein stop is supposed to be a trained thing. At least, that is how one instructor told me to use it. In most cases, I think if you have room, a spiral that slows them down is good.
Another option is the pulley stop:
I've tried it while riding with a halter instead of a bridle, and it worked OK. The advantage is a straight stop, but the horse (mine, at least) will bolt again when pressure is released. The worst I have been hurt riding was when I tried to dismount after stopping her, and she bolted in mid-dismount. I landed back first on some rocks, and my back still hurts 3.5 years later.
I refused to use it on a bolt about 6 months ago, because I felt she would topple if I did. I don't know why I thought that...maybe she felt unbalanced to my subconscious mind.
If the terrain permits, what has worked best for me is my voice. "Easy" is my verbal command to drop down a gait. Settling in my seat, spreading my knees (I tend to clutch with my knees when nervous), loose reins and softly calling "Easy" has sometimes dropped her down a notch within a few steps. Once she remembers I'm there, she calms down.
If someone has the perfect, works every time technique, I'd sure like to know it. I need