I think a combat knife should best be described as a utility knife designed for extreme abuse with consideration given to it's potential use as a weapon.
A knife who's primary purpose is as a weapon should be referred to as a fighting knife.
So, I think a combat knife should be of the "sharpened prybar" variety, capable of being used to hack into vehicle sheet metal for rescue, pry open crates, perform light breaching tasks, puncture tires, dig and probe soil, prepare food, and survive impacts with concrete and other hard surfaces. It should have a secure grip, a full tang or rear enough to ensure strength and rigidity, and be small enough to fit on LBE without interfering with any other kit. The blade should be electrically isolated.
I think the Cold Steel SRK and Gerber Strongarm are the closest to "perfection". The SRK needs a glass breaker IMO and wouldn't be harmed by a beefier tang, and the Strongarm could use a larger guard to prevent the hand from sliding onto the blade.
Now, for a fighting knife. I think the OP was talking about a face-to-face knife fight. I personally think that a double edged knife of the same general size, weight, and geometry as the Cold Steel Trailmaster with a grip that widens as it extends from the blade for better grip, and an aggressive pommel would be ideal for me.
A bowie, 9 1/2 -10 inches with a sharpened top swage, 3 1/2 inches or so, for a nasty back cut. Full tang, and no thicker than 1/4 inch, thinner if possible. Steel is a toss up between 1095, A2, or INFI for carbon steel fans, and Elmax or M390 on the stainless end. It should move light in your hand like it was part of it, balanced just in front of the guard, which should be substantial and forward swept. Blade breaker notch is optional. But that is what works for me. In the end, it's what works for you. Just be sure that it actually works.
Luckily most of us will never need to use a combat knife . I personally have the same opinion on as I do a self defense gun and adequate knife that someone is familiar with and has confidence in is better than the best fighting knife out there if the user isn't used to the weapon. I have faced someone with a knife once. I was armed with a chain dog leash . I ended up with a cut thumb that should have had 6-8 stitches and his case folding hunter he ended up with a busted hand jaw and in the pond with water temps at freezing.
I cut his belt to get the sheath for the knife once they drug him out of the water