The writing and acting on this show were almost universally excellent, but even so, I have some that made stronger impressions on me. The case where Det. Penhaligon was attacked, and there was the other attacker around at the same time, was absolutely terrifying. Both of the perpetrators here were among the most frightening I ever saw on the series.
I appreciated how the show got into the thinking of the people, so that you would know why they did these things. You didn't 'feel sorry' the way you would if something bad happened to your friends, but you learned to understand why they were so sick.