I think probably P. piraya, S. marginatus, or S. elongatus would hold the title for most aggressive. Maybe S. brandti. I had an S. compressus that was mean as a rattlesnake too, but with serras I think it comes down to the individual fish.
Really most piranhas aren't aggressive in the same way an SA/CA cichlid is. They don't "display" and only rarely do they glass bang. Piranhas are cold blooded killers. They don't have to be pissed to kill your fish. All it really takes is for the P to be mildly hungry or bored and then they bite the tail of a fish out of instinct. After that it's generally a matter of time before they finish the other fish off.
There's no malice or rage to a piranha. They kill things...it's what they do.