GoodFellas
What makes this great for me is the amazing storytelling and how it makes the movie flow immaculately throughout its whole lenght. And to top that off, it is backed up by interesting and nuanced characters played by a cast of very skillful actors who deliver really good performances. Not only that but this gives a different and intriguing perspetive on mafia, showing a group of people associated with the powerful families and how they rise to "sucess" in that business.
To sum it up, it is a very rich and enjoyable experience that is worth your time, especially if you take into account that it is not even 2.5 hours long.