Welcome!!
1. GoodFellas
2. Donnie Brasco
3. Casino
4. American Gangster (not about the mafia per se, but they're in it and the era was depicted brilliantly imo)
5. Gotti
Godfather is the best ever of course, but you asked about based on real life so I chose movies based on true events..
My list would look like this:
1. Casino
2. Goodfellas
3. Gotti (even thought it's incredibly factually wrong
4. Wolf of Wallstreet (more OC than mafia)
5. Black Mass
