Dark Eldar Wyches. They are the best assault unit, point for point. They nerf the assault power of opponents while being potent in their own right. Only those rare units with cc weapons that ignore Invulnerable saves are a danger. And let's face it, the only one that is remotely common is the Callidus Assassin. Pariahs I've only seen used once, and the Nightbringer is not only rare but extremely hard for any assault unit to deal with.