The subject line of this thread reads "...carry and self-defense." To me that means a utility/defense knife, and if you carry for both, you will be using the knife for utility about 99.9999999% of the time. So where should the emphasis on design be?
My notion of the ideal such knife has come to be a 3.5 to 4 inch fixed blade. This will be a strong knife that is light and easy to carry, and not very conspicuous. It should have an ergonomicly designed handle that provides excellent retention in a variety of grips. The tip should be roughly in-line with the median line of the knife, and should be fairly robust. It should come with a carry system that allows quick access and comfort since you want to carry it all the time and everywhere.
In short, it should look something like this:
Those are the Mad Dog Lab Rat and Rat Thing.
When a single folder is desirable, I recommend the Darrel Ralph Large Apogee, SpeedTech Synergy, Benchmade 710 Axis Lock, or REKAT Pioneer (remember the utility requirement). The Gerber Covert gets honorable mention because it is a great utility knife, but not in the same strength league as the aforementioned knives, in my opinion.
I am really looking forward to the REKAT SIFU, but will certainly carry a small utility folder along with it.
Harv