Harnesses, Haltis, and All That Walking Stuff
When we first got Blondi, a then 18 month old GSD with absolutely no walking manners whatsoever, she was what is traditionally (in Dogsey terms at least) a steam-train. Walking her was absolutely and flatly impossible. Unlike a smaller dog perhaps, being pulled along by a GSD is not only painful, but embarrassing too. Both of these things, I found, lead to two outcomes.
1. You refrain from walking so much as you know it's going to end in pain.
2. You actually make the dog worse, as you feel embarrassed, and this feeling of stress travels to the dogs senses and they "play up" all the more, as they are getting really mixed signals.
We've now tried the harness, the halti, and the leash. Here are my feelings on each:
Harness
I used this first to just make walking physically possible. The harness acted as a buffer, absorbing the pull stress so that my arms and shoulders weren't acting as the shock absorbers.
To this end, it worked, and worked well. However, there's a big down-side too. In no way does it teach the dog to not pull. It really does just stop you feeling the pull. For the most part, you'll see that your dog is still walking ahead of you.
There's also a bit more fuss putting it on, and so if your dog is one of those which gets excited at leash time, then it's likely to become more hassle as you have to wrestle the dog to put the various attachments in place.
Summary: Great for an immediate "fix" on real problem pullers, but be aware that the moment you are without it, the dog will stil be pulling exactly as it was before.
Halti
The more I used the Halti, the less I liked it. From a safety point of view, I just can't trust it. Because of the way that it holds your dog's head from the side, it's way too easy to imagine, when you have a dog that likes to lunge at other dogs, as mine does, that she turns her head one way, as I try to restrain it, and snap, some serious damage is going to be done to the dog.
She didn't much like it either, no matter how many times we put it on her, or for how long. In fact, walks became worse as she spent more time worrying about the Halti than she did the actual walk. Most certainly her attention was on the halti, and not on me, that's a fact.
Also with the Halti, you just know that it's essentially doing the same as the harness. It's not teaching your dog to walk to heel, it's just helping you to manage a problem. Though it is slightly less hassle to put on the dog than a harness.
Summary: Potentially dangerous for dogs that lunge at the other dogs. May distract the dog as they become more focused on removing the Halti than they do on you or the actual walk.
The Collar and Leash
Now that I've been shown how to handle the leash, based on Blondi's particular pulling habits, and administer the "heel" command with a slight check on the leash, I would say that the collar and leash is the simplest and most effective method, by far, though perhaps not the quickest.
It's not the quickest, because you are having to really teach the dog how to walk correctly by your side, as opposed to just disguising or managing a problem. You will, undoubtedly, need to spend several days going around and around a fixed area of land to start with, but I can say that the extra effort is really worthwhile. Particularly, if you can get a qualified behaviourist/trainer to take you through handling the leash correctly, based on your own dog's peculiarities.
For example, I was using the stop-start method which is quite common. As Rob pointed out, that was totally ineffective on our dog, as she didn't care one bit about the stops, and used them to admire the scenery. Aside from that, we were getting nowhere, distance wise.
Summary: Will take longer, but is, I feel, the overall best solution. If possible, pay to see a trainer/behaviourist for one session to show you how to handle the leash correctly, for YOUR particular dog's habit(s). I really don't believe there is a one correct method for walking all dogs.