We’ve always used the popular classic leashes you find in a pet store, the nylon kind.
Nowadays, you’ll find so many other choices: retractable, personalized, round, double, leather, short, long and even car seat belts for dogs. Whichever style you pick, just make sure it’s a strong and sturdy leash.
Retractable leashes should not be used. A dog will learn to pull which extends the rope, letting the pet get too far from the owner. This in turn could become dangerous very quickly.