XR is right, it largely depends on the individual fish a lot of the time. But in general (overall) black piranhas are certainly very aggressive.
Unfortunately I tend to disagree with Santoury to almost a complete extent. Although I agree piranhas can be skittish: they will attack through the glass, they will kill other tank mates (more so than cichlids) and they have one more thing over cichlids
. Teeth
.
And you clearly have not ever kept a pacu, because they are extremely skittish and dart at the first sign on major movement. Hence why pacu have a reputation for being true tank busters: not for being super aggressive, but super strong and twitchy, which is a combination for broken glass.
I think that black piranhas are certainly up there with the other top most aggressive fish, however few would crown them the ultimate king of aggression. Although it is debatable if any one fish is indeed the most aggressive. Dont get a rhom based on people saying they are very aggressive though, piranhas can only be enjoyed when you like their other factors in addition.