Poirot starts with a great script, the famous Poirot novels and short stories by Agatha Christie, and manages to complete it with an excellent performance by the cast and the direction.
Modern shows like Sopranos & co. can't even begin to achieve the quality that this show created. In fact, I'm still wondering what people find so extraordinary in modern shows, and the fact is, the only reason people watch them is because they´re new and they constantly break taboos, there's absolutely nothing more to them. Mad Men is horrible, Sopranos wasn't more than ok and the new "Crime Investigation" series, such as CSI are just schools for psychopaths.
Poirot, however, moved far beyond redundant entertainment and created something unique that made it eternal and gave it a place that no other show was able to take.
My only regret in regards to the cast in Poirot is that they haven't been used more often in other shows, in particular David Suchet (plays Hercule Poirot), since it is obvious he's a very good actor and capable of much more than incessant repeats and small (thirdiary ?) rolls.
I give it 10 / 10.