The Long Good Friday (Bob Hoskins and Charlie from Casualty - look out for a Small appearance by Pierce Brosnan) is one of the best, and the original Get Carter (Michael Caine) are highly recomended.
A good trick,is to find out which Real Life Gangsters starred in films.John Bindon was a notorious London gangster in the 70's,and appeared in things like The Sweeney,Quadrophenia etc.He was embroiled in the Princess Margaret Scandal in real life aswell.