Depends on the circumstance. If the hit man can get very close to the victim, a .22 revolver is the best. It's not a self-defense weapon (not enough stopping power). But it's small, concealable, relatively quiet and doesn't eject spent rounds. If the shooter can get close enough to put several rounds in the victim's brain, it's perfect. You saw Joe Pesci do this to a woman in "Casino."

But, for general protection against other mobsters, and to threaten them, they need a bigger, higher caliber gun.