I've always lived sorrounded by dogs. I love big breeds but I take their trainning very seriously, I'm not going to be one of those idiots who can't control their dog and give the whole breed a bad name. I hate hurting them, even when they attack me (weird for a guy who hunts for a living, isn't it?).
In the city I have a really large Mastiff (he weighs 100kg, around 200 pounds) a young Pit Bull and a ridiculously small mixed breed dog that just walked into my yard a couple of years ago (watching him play with the mastiff is great). Al my dogs are very friendly, they are used to other dogs and people (I try to make them play with kids as often as possible). The mastiff has a lot of training as a rescue dog. Since I usually walk them together with a friend who has six labs (also rescue dogs) I haven't had many problems with other dogs, since they tend to stay away from a tight knit group like this one.
Anyway, the only problems I've had were with owners too stupid to keep an eye on their dogs and dog walkers who just sit down at the park with their headphones and let their dogs loose. I'm proud to say that my dogs never started a fight and in many years I only had a nasty incident. There was a doberman trying to attack my smaller dog and my mastiff stood infront of him, looking at me and waitting for permission to help his little buddy. I put both of my dogs on leashes and shouted to the dog walker who was sitting down under a tree listening to music, oblivious to what was happening. Things got out of control when the doberman finally attacked the my little dog and my mastiff went berserk. I pulled off the small dog, but I wasn't able to separate the other two (plus the little one kept trying to get back in). The poor doberman got really mangled (thank God it wasn't killed and a veterinary friend of mine had his office right across the street). When everything was over the idiotic dog walker came running and threatening me to call the police because of what my dog had done. I didn't let him finish the sentence before I kicked his ass, I was so pissed of that this idiot had gotten the animals hurt (not just mine, I can't stand the sight of a wounded dog). He tried to press charges for assault, but since the park was a block away from my home lots of neighbours who know I'm not an agressive person and whose kids play soccer with my dogs told the police what happened and I didn't even have to go to the police station.
In the mountains or in the countryside dogs are a bit more of a problem. Sometimes they'll come out of a field just running at you. I think a good, thick, walking stick is an excelent defense. If I was with kids and thought there was real danger, I would act like with a mountain lion or wild boar: fill it up with lead. Extendable batons can be a good choice for urban dog attacks. I know I could take out almost any dog with my knife, but I would probably get hurt badly in the process. Heavy boots are another good thing.
In the city I find that the problems are usually the owners. People don't assume the responsability that comes with owning a dog and the poor critters get hurt because of the overpopulation of idiots.