If you are worried about encountering an off-leash dog that is potentially dangerous I suggest you carry a can of citronella spray. Basically dog mace. Spray it right in their face. If they are close enough aim for the nose & eyes for max irritation. Get it on amazon

A small air horn is another option as the sound might scare them. Have an umbrella on you? Open it up as a shield. Or grab something near by like a stick or rock.

Have a small dog? Do your best to NOT pick up your dog if the dangerous dog is close. A larger breed might see fast moving small fluffy thing and have an intense desire to get it in its mouth. But - if you are near say a garbage bin, take the chance and open it up and drop your dog in there and close the lid. If you are next to a fenced enclosure like somones yard drop them in there.

One recommendation I see a lot is to throw treats at the dog if you have any on you. If the dog is truly aggressive this will probably not work. If the dog doesn't seem to be threatening but you aren't sure of their intentions toss a handful right at their feet. It might give you some time to back away from the dog.

I've had people tell me yelling "GO HOME!" has worked for them. Again, if the dog is truly aggressive and means business that probably isn't going to work.