I cannot give the single reason that makes this series is so perfect . Sharply observed narration by David Tennant is superb and each of the actors were superbly cast . Understated stereotypes all contributed in perfect measure to execute this really witty script . The ending with Hugh Bonneville and Olivia Colman is the best ending I have ever seen, bar one (Blackadder).
Please give this a watch ! So that's all good.....