Goodfellas (1990) and Casino (1995)

Obviously these two are practically from the same director/cast. I do believe they are the very best of their genre. Personally I prefer Casino. I don't know, I just like its setting more. A more glamorous feel of Las Vegas compared to the crude New York from Goodfellas.

Chinatown (1974) and L.A. Confidential (1997)

I bent the thread rule here since their release dates are more than 20 years apart. I find them very similar and, like the previous examples, I regard these two as the best in the crime/mystery genre. Here I am unable to pick a favourite. What I really like about them is the setting, i.e. Los Angeles in 30' and 50', respectively. To me they kinda created this image of "America" as an European would imagine it.