At least in the top 10!
Capone and a few others surely gave him a run for his money, but there's no doubt that James "Whitey" Bulger left a mark and he is considered the most dangerous and feared Irish-American mobster ever. He ruled Boston from the mid-60's until the early 90's, all with the help of crooked FBI agents and local cops, as well as his younger brother Bill Bulger, who was the Speaker of the Massachusetts state senate!
Black Mass was one of the best books ever on this piece of trash, who is now in his 80's and rotting away in federal prison for killing more than 20 people and extorting dozens of others while also controlling prostitution and drugs in Boston for 25 years. The book is now made into a movie starring Johnny Depp, which comes out in the fall; here are the trailers for it.
Can't wait to see this one.