In my opinion, one of the best you can get values in a combat knife is a Cattaraugus 225Q from WWII. Here is my father's:
You can find these listed anywhere from under a hundred bucks to several hundred dollars or more on eBay. To get a good deal, you will likely need to watch for a while.
Or, you can buy just a Cattaraugus 225Q blade, or one with a broken handle/tang end. (Many people do not know how to remove the handle butt washers and wind up breaking the knife handle.) Then have a knife maker rehandle it for you:
I also have several Buck knives and a Swedish Fallkniven A1.
At one point I had a half dozen Randalls but am now down to one, a Model 3 Hunter, I think, plus a couple of handmade knives.
Randall:
I like this model, 5" blade, because I think it a practical and useful size. Plus Bo Randall said it was his favorite model and length because he could wear it and ride comfortably in his pick up truck. so it seemed the one to keep when I was selling off the others.
And an old, worn out Western knife from the late forties (IIRC) that I got from a thrift store for $30:
I throw that in the mix because it cuts as well as any of the others, to be honest. It is light, though, and hence not useful for batoning, or as a pry bar in a pinch.
I had a Randall Model 1, the original Randall, a true combat knife. Lovely knife, and better for stabbing than the others I have mentioned, although not as good as the Model 2, another great Randall which is double edged and designed for stabbing (but not good for much else).
Model 1:
Model 2:
But, when you say combat knife, whether as a collector or as a user of the knife, what are you thinking of? I think nowdays, for example, the Cattaraugus, which my dad called his combat knife, would be called a survival knife. Multipurpose. Does pretty much everything.
I would say the Randalls are the most refined, well made, beautiful knives out there that are not custom knives by custom knife makers. Plenty sturdy, too. But they won't do anything a less expensive knife can't do, so its kinda similar to buying a Rolex rather than a Seiko, or Citizen watch, in my opinion. But if you like knives, and the beauty of knives, and have the bucks to put into it, hell yes, get one!
Here's my bunch in a glass topped table in my library/man cave:
But.... one knife with which I am not familiar is Al-Mar.