I have personally had the most trouble with German shepherds.
My boyfriend rented an apartment at a house where the owner had a couple of them. One was clearly unstable. He got more and more hostile toward me to the point where one day he jumped me from behind and bit me on the back of my thigh. Didn't break my skin. After that I'd carry a big rock or stick to hit the dog if he got too close.
Once when I was on a 24 mile day hike with some friends I was bit by a part German shepherd dog on the back of my calf. Problem was we were in the Wilderness and the guy who owned the dog clearly was the type who owned a couple of guns and was equally as unstable, so I just got the heck out of there. It was 7 miles back to the car. I have a big scar.
I am scared of dogs now. I hike with hiking poles and cross them in front of me whenever I see dogs. They and their poison oak encrusted fur stay away from me. Even the friendly ones seem to know what the sticks mean.
I've never been threatened while riding a bike so far. I am sure it would be a horrible event. I would feel very vulnerable without a stick or a rock or something. I don't even have a water bottle handy to throw. Maybe I could grab my flag pole and hit the dog with it.
I have nothing against dogs, but I don't trust them anymore.