I measure aggressiveness by how willing I am to put my hand in the tank. I've kept a number of species many years ago. Here's my personal experience.
manueli - wuss
brandtii - finger chaser until you put your hand in the tank
elongatus - finger chaser until you put your hand in the tank
16" rhom - doesn't care if your hand is in the tank, just don't get too
close and your fine
shoal of pygos (rbp, caribe, piraya, ternetzi) - not much of a finger chaser, just waits for food. Will scatter when you put your hand in the tank then becomes inquisitive.
shoal of geryi - will aggressively wait for food, hovering around the surface. won't even wait to identify if it's your hand before hitting it. By far the best piranha tank I've ever had.
Light travels faster than sound. This is why some people appear bright until you hear them speak.