I saw a post above that distinguished between a combat knife and a combat-utility knife. I make the same distinction using the terminology "combat knife" versus "fighter" as do a lot of high end knife makers who work in this area. For combat knife we generally mean something that is primarily used as a camping/hunting/utility tool and secondarily used as a weapon. For a pure fighter, we mean something that you would never use for, say, prying open a crate or box. It would be fast so you can actually participate in knife-vs-knife fighting and not be too slow when blocking, and might be double edged which can limit your ability to baton. You might forgo a glass breaker in favor of being able to flip easily to a reverse grip. You might prefer tactical bolsters to hold the knife in place in your hand when your hand or glove is bloody.
I saw a great video on this forum about a small 4 1/2" blade knife an ex-soldier liked a lot as a combat knife that was lightweight and would fit conveniently on his vest. Like a pry-bar police tanto I designed, of design necessity the point of balance was back into the handle rather than at the front bolster even after the skilled custom maker did the tricks. I can flip my tanto, but for optimal flipping you generally want a point of balance at the front bolster.
I was so enthused with the recommendation of one of the knives in this thread I went out and bought it. While it was a great camping knife and high value proposition, the handle and rubber like handle material are such that a normal to large sized handed man cannot flip the knife to a reverse grip and back with any degree of speed (if at all). That disqualifies the knife for hand to hand combat against a semper paribus opponent as it is not either a long-bladed sub-hilt or 12" bowie intended to fight from only one direction and long enough to do so. Further, the point of balance was too far forward to move the knife quickly which means if you have to actually block an opponent who has a knife you are going to be too slow. While this could be a great "combat knife" if you never actually have to fight knife vs knife versus an opponent with the same training as you and a knife designed to accomodate fighting, I think it is a bad choice if the intention is to actually ever fight with the knife (not the case for individuals at less specialized military levels of training). Plus, the coating it used will generate too much friction for an easy penetration.
You can produce a knife with the same length and strength that is balanced for fighting and still can serve as a rough utility knife -- I have one by Chris Reeve. Of course, it costs about $150 more by the time you are done buying a spec ops sheath for the other knife.
There are a number of more highly trained military personnel who actually use their knives to fight with, including sentry removal. Their combat knives need to actually be usable for fighting as well as prying.