Well, of course this can be difficult, I can make an impulse type of list.

1. The Godfather Trilogy
2. Road to Perdition (may seem pre-mature, but Mendes was great).
3. Bugsy
4. Goodfellas (seems to get even stronger in parts each time I get to catch it).

I also love The Untouchable, but I don't consider that a crime film, I consider it a police epic like Spartacus meets The Godfather kind of thing. The Sopranos also rules, as a TV show.