Mean Streets is less polished and more heart felt film than the other two mob films, and that's why I enjoy it the most. It's also is less complementary to the whole mobster lifestyle, than the others. I tend to like my crime films more on dealing with the fringe lower levels of crime, urban decay, etc....
I also enjoy Keitel in it, when Keitel is on, he is every bit the equal of Deniro, and Pacino. The whole interplay he has with himself in Mean Streets is wonderful. The flawed man dealing with the realities of life in that neighbourhood and circle of friends, etc....
It gets extra points for being a Ny crime film based in the 1970's, that was actually shot in the 1970's. It's a small thing, but it makes it more enjoyable. You don't find yourself looking for flaws in scenery, etc....
Finally the poster art is top notch -

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