Personally I would carry a heavy golf umbrella. You can open it to block eye contact between the dogs and get some shielding for yourself. It would buy you some time for help to arrive. You can also use it to whack a rude dog.I'm thinking a riding crop might be an easy carry. I've hardly ridden, and never had one in my hands. Those of you familiar with them, would they have enough wallop?
My second choice would be a heavy hiking staff. I would want something heavy enough to put an aggressive dog out of commission when I swung it. See my post in Vita's thread to understand a bit more about my position here.
Regarding any kind of horse whip, I think they will only be useful against rude dogs. An aggressive dog in a full out attack has its brain shut off. It won't notice the pain, or the pain will only add to its frenzy.
If you do want to carry a horse whip to deter rude dogs, I would choose something longer than a riding crop. Keep some distance between you and the dog. A dressage whip is about three feet long. A carriage whip is five or six feet long with a short lash. A lunge whip is similar, but it has a very long lash. I wouldn't recommend a lunge whip, because the lash will just get tangled on things. At this length, though, you might as well just carry a staff or an umbrella.