Two, how emotionally real the show can get. The relationships between the people, between the races, between the sexes, between the cops, is beautifully done. Top drawer. Andy's speech about Bobby, how he never thought he'd ever have a friend again, just the best you'll ever see.
Three, the single best piece of film I've ever seen is the scene in the last season where Bobby's ghost comes back and has a short conversation with Sipowicz in the bathroom. Stunningly good scene, best ever.
But every minute of every episode is spectacular. No bad moments ever. How you keep a series on such a consistently high level is beyond me.