My preference for a "combat" knife
When I was heading to Afghanistan I kept thinking of that scene in "Band of Brothers" where the German kills the Ranger with a knife in that upstairs room. As unlikely as it would be that even as an infantryman I would find myself in a similar situation, I still felt that I ought to have a decent knife with me when out on missions.
I ended up choosing CRKT's (Columbia River Knife and Tool - I'm an Oregonian, after all) CR-2705 First Strike tanto with a plain edge, blackened metal, and a sharkskin/cord grip. I liked it because it was sturdy but had a flat, low profile that wouldn't stand out too much on my IOTV.
Initially I fastened it to the back of my IOTV waist belt, but after being in the scrum of a bunch of Afghan kids I decided I preferred it in the front, fastened horizontally on my strong-side mag pouches with handle to the right. It's still there. I wrapped the sheath and the grip with ACU camo cloth to make it less noticeable.
I note that it's been discontinued, but it seems you can still get them at various places for $40 to $60.