I'm going with the Godfather, I loved the way Michael removed the heads of the other families, and Moe Green then dealt with his brother-in-law.  Today we take care of all family business.   

Maybe it doesn't technically qualify since it's not the Italian Mafia (Russian) and the mafia aren't the protagonists but The Boondock Saints is a cult classic.

I re-watched Seven Psychopaths the other night. Not a typical mobster movie but highly entertaining. 

Goodfellas 

A Bronx Tale

While not a movie - I think it still deserves mention - The Sopranos.

The Godfather 1 & 2

A Bronx Tale

Goodfellas

Casino

In all honesty my favorite mafia would have to be the Godfather. Now Goodfellas is a pretty close runner up if you know what I mean.

The Godfather is probably the greatest mob movie of all time, but there are some other good ones that I enjoy as well. The Bronx Tale is probably my second greatest mob movie. It's just something about the story that grabbed my attention. American Gangster was a good one even though I don't know if you can really call it a mob movie. A older movie call Hoodlum is another good one...if is a movie about the black mafia working with the Italian mob.

Can anyone really say there's a better one out there than the Godfather?

There’s a few great mob movies; Casino, Donnie Brasco, Goodfellas and more recently - The Irishman… essentially anything with Robert DeNiro and Joe Pesci, but the best mafia movie for me is still the first Godfather movie.

Goodfellas is a great movie. The actors growing up and we rising to status is a great touch. I loved Casino as well and cant forget  the godfather franchise. 
 

The 1996 film Gotti is absolutely amazing. By far one of my favorite mafia movies. SofiaAccardo and I have been rewatching it quite a bit for inspiration on writing. It's free on youtube. Do yourself a favor and give it a watch if you are into mafia movies. Armand Assante becomes Gotti in this film. He plays the role perfectly so perfectly. 

https://www.youtube.com/watch?v=Zh-xHOtURxc

Just skip the new Gotti and go for this. John Travolta is no Gotti. 

I don't think you can get a better mafia film than the original Godfather movie. It's a work of absolute genius - better than Godfather Part 2 in my opinion.

the entertainment of the last few hours tells me, this is the best mafia movie ive seen. Some people are completely unhinged out here.

I have to say, all of the Godfather movies, The Departed, Donnie Brasco, Casino (was awesome) Goodfellas all of them are amazing but the one I like that some people sleep on is A Bronx Tale, that was an awesome movie about the mafia as well.