Whoa -- let's take a step back here.
Pardon our bluntness, but if you're asking that question, you probably don't understand the principles of lethal force and personal defense. Let's get a few things straight.
First of all, knives are tools. Like other types of tools, certain knives excel at specific tasks -- hunting, utility, bushcraft and so on. And yes, some knives are purposely designed to be defensive tools.
Second, it's virtually impossible to avoid encountering laws and restrictions governing the carry of knives -- and a great many of those laws make a special distinction between knives intended for general use and those designed only to be deadly weapons.
Merely carrying a self-defense knife without acknowledging those two points can quickly land you behind bars. And if you actually defend yourself with such a knife -- even if your action is somehow justified -- you're more likely to face civil litigation as well as criminal charges.
Our third point is, quite simply, this: The ability to use a knife for self-defense doesn't reside in the knife -- it's a skill. Practiced well, it requires physical fitness and a disciplined mindset. Learning this skill should be done under the supervision of qualified instructors.
So we're not going to recommend a self-defense knife to you. If you're still interested in learning more about the skill and discipline of defending yourself with an edged tool, we encourage you to seek out professional guidance -- not videos on the Internet, but personal instruction in your local area.