Way back when I was a kid in the Boy Scouts we were hiking on the Appalachian trail and in broad daylight were attacked by a pack of feral dogs (like 8 or 10 of them). We fought them off, but they attacked again multiple times and even during the night too.

I have come across wild boars, black bears, cayotes, a wolf and a mountain lion, but none were as aggressive as the feral dogs.

The mountain lion was the least aggressive as it just turned around and left. Oh, and the wolf just looked over at me and continued on it's way.

Another time I came across a pack of feral goats on a trail on the face of a very steep slope--more like a cliff. I came around a bend and saw them about 100 yards away with the billy leading them. A few days prior a billy had attacked the open jeep I was in and kept ramming it, so I knew they were aggressive. However, the goats were between me and the only way out. So, I threw up my arms and went running and screaming right at them as fast as I could. Lucky for me, the billy turned around and couldn't go back fast enough. He almost knocked one of the other goats off the trail in his hast. Whew, that could have turned out bad for me.

So in my experience the ferals are the worst.