Goodfellas was one of those rare movies where the whole ensemble (from the director all the way down to the bit players) just jelled into something more than the sum of it's parts. It was so great that you forget that you are watching a movie. You feel like you are drawn into their world, and yet you have to keep reminding yourself that it's just a movie. On some level it scared the hell out of you.
Even then, you can't forget it, no matter how hard you try.
I still think the Godfather movies were the best, because they were the first, and there was more of them - it had a whole epic generational story. They were also more storybook fantasy with characters like you wanted them to be. It was Cinderella with guns.
In Goodfellas, no one lived long enough to make a generational movie. It was gritty, personal. The direction and the actors were far superior to any gangster movie ever made. It was like seeing a fantastically fatal car wreck on the highway. You didn't want to look, but you had to. Around every corner something happened that questioned your view of humanity. It was Dante's inferno with good music.