I only really know about Chicago. Here, those with the worst reps were: Vincenzo Ribaldi aka Jack McGurn (basically the Nitti character in the Untouchables) and I would say from the more modern era Sam DeStefano, Harry Aleman, Frank Schweihs and Tony Spilotro (Nikki/Pesci from Casino).

In terms of cruelty/craziness, DeStefano and Spilotro had the worst reps.

In terms of body counts, just a guess, but maybe McGurn and Aleman. In both eras they were knocking people down left and right all the time.

More recently Frank Schweihs had a real bad reputation. A few guys still walking around I won't mention..

In terms of who you wouldn't want to cross... none of them. But generally the stupider ones were the ones known for kicking your head in if you accidentally bumped into them at a nightclub, not so much the known guys, but the small timers.

I don't know much about the east coast but Roy Demeo (sp.) I think is suspected to have one of the highest body counts in American organized crime, someone can correct me if I'm wrong.