Simple really: The longest blade that can be easily concealed. Acute point with no finger choil much preferrable, double edged or single, hollow ground to slice deeper and challenge grabbing, but, most importantly,
as little edge belly as possible (edge bellies typically greatly open the edge bevel, and make poor tip slicers whether they open the edge bevel or not): Slicing with the tip gives a large reach advantage vs stabbing, since the arm can be used extended...
So as straight an edge as possible (to be closer to "tip-hooking") is crucial (meaning a heavily dropped point on a single edge), no finger choil, and hollow ground with 0.5 mm thin edge: It is surprising to see how few knives will match all these very basic recommendations...: The Al Mar "Special Warfare" is just about the only one I know to be done properly on all counts: The tip is on the fragile side, but not inappropriate for a true fighter.:
Al Mar also made the best factory dagger, called the Shadow IV, a whopping 7.75" blade for barely 10 ounces... The sharpest-edged dagger by far though is Randall's peculiar Tom Clinton special, which out-slices just about any single edge knife, but has large finger choils...
Gaston