Here are some things to consider when looking for a self defense folder:
(1) Blade length: You don't need to go crazy with it, but for a folder, 3.5" to 4" give or take is pretty ideal.
(2) Strength of the lock: Not all locks are created equal. One part is the quality of the manufacturer of the knife, and the other is the inherent strength of the type of lock. You want a good sturdy lock that won't fail or close on you. Some examples of strong locks are in no particular order:
(a) Benchmade's Axis lock
(b) Frame lock or Reeve Integral Lock (usually titanium)
(c) Spyderco's Caged Ball Bearing Lock
(d) Spyderco's Compression lock
(e) Cold Steel's Triad Lock
(f) Liner Lock-This one is iffy. Like I said, not all locks are made equal. There are really good liner locks out there and really bad ones. Good ones come from the well known makers like Spyderco, Emerson, Benchmade, etc. So make sure you know who you are buying from. A perfect example of a liner lock done right is Spyderco's Military.
(3) Speed of deployment: How fast can you get the knife from the closed position in your pocket to in your hand and locked. Something you won't be able to tell until you handle a particular knife but most modern folders with a lock are extremely fast (as fast or sometimes faster than an auto).
-A point to consider here is whether you prefer a thumb-stud opener or a thumb-hole opener. It is entirely preference.
-Noteworthy here is Emerson's patented "wave" feature that catches the fabric of your pocked and opens as you pull the knife out and -->
Spring assisted openers.
(I've found both the "wave" feature and spring assisted to be unnecessary.
(4) Comfort of carry: Especially if the knife is primarily for self defense, it will be in your pocket a lot. So pick one that isn't too heavy or uncomfortable.
(5) Blade Edge, Geometry, and Steel: Blades come in all sorts of shapes and grinds. People all have their various theories on which is the best for SD, but truth be told there are plenty of good ones.
This is about the plain edge (non-serrated), vs. serrated, vs. combo edge debate. As a practical matter, who knows if it will actually affect anything, but I follow the esteemed Michael Janich's philosophy of thought that plain edge works best for self defense. Why? Because serrations tend to catch and get stuck on clothing.
As far as blade shape, there are many good ones and no "best." Some popular shapes are clip points, bowies (type of clip), drop point, spear point, tanto, etc. They all have a trade of some sort. Find one that appeals to you.
Blade Steel: Not too important if all you're going to do with the knife is carry for SD. There are myriad good steels out there, any of which would be fine for SD. For folders there are two main types of steels: stainless and tool (carbon) steel. Generally, but not always, stainless will have better corrosion resistance, but tool steels will be tougher/less likely to chip, break, whatever. Something to remember about steels is there are ALWAYS trade offs. Some examples of good stainless steels are 154cm, S30v, 440c, and S90v. Some examples of good tool steels: D2, CPM-M4, and M2.
(6) Ergonomics: This pertains to the general features of the entire knife. Things to look for in SD oriented ergos:
(a) Grippy handle. You want the handle material and texture to have good grip to it, even when it's wet with blood, water, etc. Good examples of high quality handle materials that are typically grippy are G-10, carbon fiber, micarta, titanium (to a lesser extent).
(b) Ramps and jimping: You want the knife to have some kind of hand-guard or thumb ramp so that in a thrusting/stabbing motion your hand doesn't slide over the handle and onto the blade. Jimping is a term for rounded metal serrations around the knife to enhance your grip. Not a must, but something to consider.
That's all I can think of at the moment. So in closing to that thesis I'll list some makers and models that I and others think are good SD knives. P.S. A lot of the top companies have SD specific models.
List of high quality manufacturers:
Spyderco, Benchmade, Emerson, Kershaw, Zero Tolerance (Kershaw's sub-line), Chris Reeve, and Strider. Be aware that Chris Reeve and Strider are $300 and above new.
List of potential SD models for you (these are just some examples, shop around online):
Spyderco's: Military, Manix 2, Paramilitary 2, Chinook 3, Civilian, Endura (the Civilian and Chinook are specialized self-defense models. The Civilian was designed for undercover law enforcement with no knife-fighting training).
Benchmades: 710, Griptillian, 950 Rift, On-Slaught, Beldam,
Chris Reeve: Large Sebenza 21, Umnumzaan
Strider: SMF, SNG
Zero Tolerance: Pretty much everything in their line is good for SD.
As to the other manufacturers, I don't have much or any experience with them so I'm not familiar with their models but I listed them because they are generally accepted, high quality brands among the knife community.
Hope this helps. Also, don't listen to people who say knives are worthless for self defense. How do you think people killed each other for thousands of years before guns? Knives are definitely not the best choice for something to defend your life with, but sometimes that's all you have. If you have anymore questions feel free to email me at email@example.com
. Take care.