actually laughed out loud when I saw the title of this thread. I love it.

IDK about the BIGGEST of ALL-TIME but a few that come to mind...

Tommy D. was supposed to be a pretty big scumbag. Was a noted rapist and a general loose cannon. Ran with some killers and junkies, was never seemingly respected or liked just feared for his temper.

Tony Mirra is another one. Maybe not as high profile a choice as you may have hoped for but Mirra was truly despised. The guy earned very well when he was on the outside, that being said he openly as unwelcome as anyone could be in the mafia while still being involved.

I've always said Casso just because of how unnecessary it was to double cross everyone who worked under him while he was on the run out of pure paranoia. Chiodo was never gonna rat, D'Arco was never gonna rat. I think having Eppolito and Carracappa was a double edged sword that saved his life a few times but definitely got in his way in several others.

Sam DeStefano is another one. Quality throwback Chicago guy who was an animal. Killed his own brother when it could have been worked out otherwise. Kidnapped the first black guy he saw on the street after an argument with his wife and brought him home to rape his wife at gunpoint.
Was a noted psycho and not well reputed on the streets. After he was killed even for being one of the top earners of his time, his funeral was scarcely attended.

Frank Schweis is another Chicago guy from the next era after DeStefano. He was an actual monster.Killed everything that moved, was the most feared guy in Chicago for decades. Whenever a racket was restructured, Schweis would handle logistics and he was absolutely one of the best ever at that. Not sure anyone terrified people quicker when they walked into a room in Chicago for about a 50 year period.