Obey your state and local laws. Also, the most effective weapon is one you know how to use and are well trained in. If you can't hit anything with a 10mm, 0% accuracy is far less effective than 90% accuracy with a .22. In any situation where you may be faced with lethal force, the weapon you are best trained in is the best one to use.

Do you know what the best defense is, though? Not getting involved in riots and putting yourself in that situation to begin with. There are very few states in which inserting yourself into a riot, and then firing at people in "self defense", is going to be seen as self defense. This is where knowing your state and local laws come in, and being proactive in ensuring you deescalate and remove yourself from bad situations before they become worse situations. In many states, you have a duty to retreat, and you are legally required to deescalate, just as the police are.

It goes without saying... do not illegally carry any weapon or other "self defense" item, in some states armor. In summary, don't be an idiot. If you cannot be a mature bigger man and have no training on how to handle mob situations, you're far more likely to kill innocent people with a pea shooter than realistically defend yourself. Especially in crowds. And most especially with people in said crowds who can possibly reach for said weapon and cause a discharge.