If by greatest you mean the most successful and smartest, or the one that managed not to get caught, TB would no far better than I.

However, IMHO, just by popularity or most well known, especially to those who are not into the "gangster" era, I would guess Al Capone is the most famous. Most everyone has heard of him.

TIS

"Mankind must put an end to war before war puts an end to mankind. War will exist until that distant day when the conscientious objector enjoys the same reputation and prestige that the warrior does today." JFK

"War is over, if you want it" - John Lennon