What to look for
Length: 4 to 6 feet for training and neighborhood walks. Short enough to manage, long enough for the dog to sniff and make choices. Longer lines (15 to 30 feet) are excellent for recall practice in open areas, but that is a separate tool for a separate job.
Flat and comfortable. Flat nylon, leather, or biothane (a waterproof coated webbing) with a padded handle if your dog pulls. Rope-style leashes look nice but can burn your hands when a dog lunges.
A solid clasp. Bolt snaps are reliable and easy to clip one-handed. Check the clasp size against your dog's collar or harness ring; a giant clasp on a small ring is annoying every single walk.
Learn to hold it right. Loop the handle over your thumb, close your fist, and gather the slack in loops in the same hand. Never wrap the leash around your hand or wrist; a sudden lunge can injure you. A second hand on the leash gives you steering for tight spots.
Match length to context. The 6-foot leash is your daily driver. For vet visits, cafes, and crowded sidewalks, choke up to 2 or 3 feet of working length. For recall training in a field, switch to a long line. One leash cannot do every job, but two cover nearly all of them.
What to avoid
Retractable leashes for training. The constant tension teaches the dog that pulling is how you get more leash, which is the exact opposite of what loose-leash training teaches. They also give you poor control near roads, other dogs, and strangers.
Chain leashes. Heavy, noisy, and unpleasant to handle. There is no training advantage to make up for it.
Bungee leashes for training. The stretch feels nice on your shoulder, but it teaches the dog that steady pressure is normal. For loose-leash training you want the dog to feel the exact moment the leash goes slack, and elastic hides that information.
Leashes with no handle. Some minimalist designs skip the loop handle. In an emergency you need something to grab fast. Always have a real handle.
Check the hardware yearly. Clasps wear, stitching frays, and a leash failure at the wrong moment is dangerous. Retire a leash at the first sign of fraying, not the tenth. A leash is safety equipment first and a training tool second.
When the old leash retires, keep it as the backup in the car. The spare you never need beats the walk you have to cancel. Inspect the clasp spring too; a sticky snap is a failure waiting to happen.
A very short leash as your only leash. A short traffic handle is fine as a second handle for tight spots, but a 2-foot leash gives the dog no chance to practice making good choices.
Our take
One flat 6-foot leash covers almost all training. Add a long line later for recall work in open spaces. Skip the retractable leash entirely and your loose-leash training gets easier immediately, because you stop rewarding the pulling you are trying to fix.