Jumping into this topic a little late, but I think it's a great question.
As many of you have said: Brain, Awareness, Cell-Phone... etc etc etc etc. Yea, that's cool. But it's a safe answer. We all have those.
I have personally been thinking a lot about personal weapons recently, especially since I actually had to pull my knife recently (second time in my life, first one was over 10 years ago.) I didn't have to open it (which I'll explain in a second), but it was in my hand and ready to "go".
I personally carry a 4" folder (legal in CT). My favorite is the Emerson Combat Karambit with the wave, but I have noticed people tend to notice it more due to the profile of the knife. My EDC is typically the Cold Steel AK-47.
But due to my recent experience. I have a few thoughts on knives and personal weapons.
1.) I have spend a lot of time training on deploying my knife. I understand and am confident getting it into my hand the way I need to. So, I think that if you are going to carry a knife, this is one of the most important things. Being familiar with your blade.
2.) You don't need to actually deploy your knife to make it effective. It is an excellent blunt object similar to a kukaton or other small blunt object. Along those lines I decided to buy a smith and wesson tactical pen. This is a nice option as well.
3.) I don't necessarily need to carry a knife, a tactical pen, kukaton or other. But, I am happy that I do.
4.) Flashlights are probably my favorite.
When I worked security I also carried the Surefire E2D Defender flashlight and own a few of these. (Bedroom, Car, & Carry). The profile of the light is hard to carry in your pocket. But, it depends on what I wear or where I am going I use it. I traveled to Spain recently to Valencia to train, and this is what I brought with me.
I really like the flashlight because it's extremely bright and when shined in the eyes is causes disruption, quick blindness, and can also be used as a blunt object. When working security the flashlight was an awesome option because it seems to just "stop" people, cause them to put their hands up (to stop the light in their eyes), and changed their "mood". I NEVER actually had to hit anyone with the flashlight after. Some would get pissed, but they never decided to take it any further.
When I pulled my knife a few months ago, 2 people in masks and large camo jackets came walking down a long private driveway into a private party. It was a couple of weeks before Halloween. The didn't say a word and kept advancing. I interjected. Long story short, it turned out to be the drunk brother of the host and his friend (they weren't supposed to be there). They thought they were being funny.... it wasn't. Looking back - I am glad I didn't open the knife, but glad it was on me at the same time. Putting it in my hand and advancing on them, changed their mood quickly.
In my experience, when alcohol is involved, people tend to go for their knife or weapon much more quickly. Booze breaks down inhibitions which can make it fun when talking to the ladies or but bad when violence makes an appearance. Side note: I was not drinking that day (driving) and glad I had the wherewithal to not actually open the knife.
Lastly, I am a huge fan of weapons of opportunity. Fight "levelers". I always ask myself: "What can I smaller, weaker person do to level the fight if attacked by a larger person or a group of people?". Depending on what you have available, there's a lot! It's just creativity and understanding that what you carry can be a weapon. Purse, Pen, Backpack, Keys, Wallet, Phone (weapon), Bottle, Chair... .the list goes on. Things can be used as distractions, objects to strike with, or both. Again, I'll go back the flashlight because it has both. It can distract / deter your attacker with the light and hit them with the blunt object.
So - I think there is no right answer to "what is the best weapon". It's what you are comfortable with, combined with your training and combined with your mindset. If you are someone who parties a lot - I would NOT suggest carrying a knife. If your job requires you to be in difficult situations or has you traveling to places that are difficult, then carry something that is appropriate for that situation. This is not the Zombie Apocalypse - yet
So there's no need to be armed to the teeth. Carry something that you know how to use, is effective, but also not truly deadly.
Thanks, Matt