A flat collar's real job is carrying identification. It is not a training tool and it is a poor place to attach a leash on a dog who pulls, because everything the dog does goes through their throat.
Nylon webbing with the dog's name and your phone number embroidered directly into the strap rather than printed on a tag that wears blank or falls off. Twenty-eight size and colour combinations. Metal buckle and D-ring.
Why embroidered rather than a tag
Tags are the first thing to go when a dog gets loose in undergrowth, and that is exactly when the information matters most. Stitched lettering stays legible for the life of the collar and makes no noise, which some dogs and most owners appreciate.
Fit
Two fingers flat between collar and neck. Tighter and it rubs, looser and a panicked dog will back straight out of it. Check the fit monthly on a growing dog.
Include your phone number rather than only the dog's name. Whoever finds them needs to reach you, not greet them.