LETHAL WEAPONS
Police and security guards use guns because they are the safest and most practical weapons of last resort against offenders armed with deadly weapons. They are a very effective deterrent as most offenders will flee or surrender when confronted with an armed 'victim'. Modern guns designed for defensive use are best (not single-shot sporting guns). Knives and large police style flashlights can be useful but any contact weapon is less than ideal and of little use against a gun toting felon.
LESS-LETHAL WEAPONS
Twelve gauge shotguns can be loaded with less-lethal bean bag type rounds like the 'Flexible Baton' (a nylon bag filled with lead pellets). They are normally meant to be fired at the offender's gut area in order to wind them, and are unlikely to cause a life threatening injury unless aimed at the offender's head or throat (only when justified in using lethal force). They can be very effective but light cover or insensitivity to pain due to intoxication can render them impotent.
NON-LETHAL WEAPONS
Oleoresin capsaicin (OC) pepper sprays are safe and very effective on animals and lowly motivated human offenders like drunks, but are restricted to occupational users in many areas and not suitable for use in windy conditions. The mist type are best as they allow users to place a wall of irritant between them and the assailant as they retreat. Tasers (dart firing stun guns) are safe and can be very effective, but are restricted items in most areas, have a short effective range, are slow to reload (can be used like a contact type stun gun) and cannot penetrate light cover. Older style stun guns are unreliable and can only be used as contact weapons (inherently dangerous as this places the user within reach of the assailant). Pain compliance tools like Persuaders/Kubotans and collapsible batons are not suited to defensive use.