I’m a big fan of ARs, because they offer the best damage per AP/accuracy combination. I favor the Deimos and the basic AR because they have higher accuracy. The problem with Piranha is the low accuracy, which means you have to be close to the target. But if you are close to the target you can aim at the body part with less armor, so you might be better off with a Deimos anyway.
Another thing to consider is that there is a bug that increases armor shredding, making all the ARs except Piranha much more useful. Laser Rifle armor shredding