I can't name just one movie...impossible.
Movies...good movies...are art. What art I like best at any given moment depends on my mood, what's going on in my life, many other variables.
No one movie covers everything. Today's favorite might be a comedy, tomorrow a character study, then the next day an over the top action flick just for fun.
I like gangster movies too. Goodfellas is a great movie. But its hard to top Godfather I and II. Any of those three I can watch over and over. I enjoyed American Gangster a couple of months ago. I wasn't sure I'd like it going into it, but I was impressed and enjoyed it.
I love the classics. I probably watch TCM on cable more than any other channel on TV. I DVR old movies and watch them later. But I like new movies too and movies that are classic now but were fresh and new when I was younger.