Carlo gambino comes to mind because of his longevity, but in my humble opinion, the smartest gangsters are some people we barely hear about. Some soldiers who made their fortunes and avoided promotions and heat. If I could be a button man who just runs some gambling and whatever else, but stays out of top positions and internal battles for control, I would opt for that approach. People who can balance their greed and ambition and make it out of the life without a Wikipedia page are the smartest ones out there