What did you like in the Godfather Trilogy?

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I recently had the opportunity to see the Godfather Trilogy(3 parts each of 2 and half hours at least). I liked the movie as every did. But it is something that I don't understand, what is something special about this movie. Alright the screenplay, storyline, camera work everything is good. So is in someother blockbuster movie. My question is why is this movie at the top of the rankings? Both part 1 and part 2 are in the top 250 of imdb. What do you think is the reason for it? Something that is special, and lacking in any other movie.