Ok, I just found this Weapons and Tactics Discussion area, and knives happen to be one of my areas of expertise. The truth is these types of combat knives are subcategorized into primarily three areas, and all are going to be fixed blades for the sake of discussion.
Combat Killing Knives: These are the longer 6+ inch double edged dagger type blades, similar to the now discontinued Camillus Cuda, or Bob Tetrozula style blades. Their primary function is for tasks like enemy sentry removal, or knife fighter to knife fighter combat. (These would be essentially useless as an all around combat/survival knife, as their blades are a long thin profile intended to reach vital organs and not designed for utility tasks).
Combat Fighting/ Utility Knives: These types of knives generally have a heavier profile 7+ inch single edged, plain or partially serrated blade. The "Ka-Bar Knives" fall into this category and are now made of 1095 CroVan or D2 Tool Steel. The Ka-Bars are a great bang for your buck. At the absolute top of the food chain in this category are the "Randall Made Knives, Model 14 Attack Knives", or the hard to find Israeli "Dustar Model 1", in D2 steel, that comes out of a small shop in Tel Aviv. The "Chris Reeve Green Beret" or "Chris Reeve Neil Roberts Warrior Knife", are also excellent knives and both are made of CPM S30V.
Combat/ Survival Knives: These type of knives generally have a heavy profile 5-6 inch single edge blade with an accent on the Military SERE survival function. Sheaths generally have an accessory pouch, built in sharpening stone feature, etc... Some examples of these would be the "Gerber ASEK Knives". The top of the food chain here would be the "Randall Made Knives, Model 18 Knife", or the "Chris Reeve Mk IV, V, or VI Knives" in A2 Tool Steel.
I would avoid most of the cheap "Rambo Type" knives that you see, unless they are from a respected knifesmith or maker.
Just a quick note: Check you local laws. Most double edged knives are considered daggers, and heavily regulated or prohibited for carry purposes, and most states have a 5.5 inch blade length limit for carry knives. Some states have exemption provisions on length, if you are hunting, chopping weeds etc..., but be sure to check and be safe rather than sorry. I hope this helps. Good luck!
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