The most aggressive chicken breed I've came across was the Dark Cornish chicken breed which is very protective of it's flock and will peck at you or run at you to scare you away.
On my Grandmas Farm I remember when I was 5 years old my Grandma has some Dark Cornish Chicken breeds and they would flock me for basically no reason.
I hated those chickens and all I wanted to do was be friendly with those chickens and play with them but they wanted to attack me and left scratches on me.
There are several other aggressive chicken breeds and some roosters can even be aggressive as there was a rooster at my Grandmas that would chase after my mom and she would run away and it was hilarious.
Most chickens are pretty good though and not too dangerous but they can attack when they want too or need too.