Aliens are nothing without a host. That's their greatest weakness. If it wasn't for a host they'd just be eggs...sitting on a derelict ship for who knows how long.
I definitely say Predators. As far as we know nothing can compete with them. Their drive for hunting bigger, and more dangerous game sets them apart from every other race. I like to think that Predators started off as primitive beings and gradually hunted other races and adapted/stole their technology, while still keeping their primitive roots. Hence the combination of a primitive being with highly advanced modifications. It's an awesome combo.