A regular trailer tie with a bull snap on one end and a quick release snap on the other. The bull snap does on the Halter and the breakaway snap to the trailer tie.
STAY AWAY from those elastic style ties. a horse can break them and the fly back when stretched to the breaking point can seriously injure you or the horse.
"The best things in life are nearest: Breath in your nostrils, light in your eyes, flowers at your feet, duties at your hand, the path of right just before you. Then do not grasp at the stars, but do life's plain, common work as it comes, certain that daily duties and daily bread are the sweetest things in life. " Author: Robert Louis Stevenson
We use Blocker Tie Rings (also sold by Clinton Anderson as the Aussie Tie Ring).
You just fold the lead rope into a bend, pass it through the ring and engage the catch on the back of the ring. This creates a sort of friction hold on the lead rope that will allow the rope to slide if the horse were to fall. A very quick to tie and release solution the doesn't require any kind of extra hardware and quick release snaps.
These also function as a training device for horses that don't tie well and pull back. The horse can pull pack just a little until he finds his own release and doesn't feel trapped.