Just a thought.
Pepper spray (OC) is essentially irrelevant once you're in close. If someone wants to kill you, and you pepper spray him, not only will it also affect you, but he can continue the attack. (I've been pepper sprayed during classes a few times, and it really sucks, and continues to suck for a long time, but I was able to unholster and fire into a body-sized target at 3 meters with all shots on target).
Tasers are ineffective if someone is wearing heavy clothing. They're also single-shot -- if someone runs at you, and you miss with the taser, you're kind of screwed. They're probably the closest, though.
Contact stun guns are pretty horrible because they require you to be in close. Essentially, if I can win a fight with someone to the point where I can use a stun gun on him, I'd just skip the stun gun and stomp him. They're really useful in psych wards or other environments like that to control people who you could actually control otherwise, while doing less damage to them. Pretty irrelevant for self defense.
Knives and sticks are potentially quite lethal, and are ineffective unless you're highly trained or substantially overmatch your opponent in strength/skill/size.
Flashlights (the 100+ lumen combat type) are probably the best in that they have low downsides, and are fairly effective in defense, and can be used to identify threats. I'd never carry a pistol without a flashlight in my pocket, and all my home-defense guns have mounted lights.
Sound is an interesting idea (infrasound, in the sub 10hz range). There's stuff like the LRAD. Unfortunately it is the size of a truck.
http://artofmanliness.com/2012/11/07/how-to-use-a-tactical-f...
I might buy a high-powered flashlight soon, then :)