Yes, the best tactical knife is the one you have with you. But that is only half the issue and not the useful half at that. What criteria should one use in selecting a custom knife to carry when you have a self-defense role in mind for it? Maybe we can come up with a slightly more illuminating answer.
I actually agree with those who say, don't bring a knife to a gun fight, though the statement is rarely helpful. If you truly anticipate the need to use deadly force in self defense, you should probably carry a gun if legally possible, or make plans to change your life style before it gets you killed. But just assume that like me and a lot of other people, you carry a knife all the time anyway, so why not carry one that won't fall apart on you in the rare and really unexpected event that you need to use it in a self-defense situation.
The best custom folder for self defense will have a strong blade with a strong tip, and a strong pivot. It will have a strong and reliable lock, and a handle that offers a very secure grip in a variety of positions. It will be easy and fairly quick to deploy under difficult circumstances. And, yes, it should appeal to you, fit your life style, perhaps be suitable for all the little cutting chores that you will do mostly with it, so that it is with you when you need it. What the knife is made of, what it looks like, etc. are not very important, just that it is strong, quick to deploy, and easy to hold on to.
There may be a lot of knives that fit that description for you, and they may be completely different from those that fit the description for someone else, so it is a little hard to say what the best custom folder is for self defense. I personally look for knives with integral locks, rather than liner locks, because then tend to be more reliable. Other than that, anything is strong, easy to hold on to, and that you think is really cool might be the best for you.
Then again, you could always carry a damascus, liner lock, jewel studded trinket and a .45 if you truly need self defense, and your state allows you to carry a firearm.