The Randall Model 14 was the most popular among several Army Special Forces men I asked about knives.
No one has mentioned the Fallkniven A-1 and S-1. Look them up on the Fallkniven site:
www.fallkniven.com
You can buy them on Amazon and other US sites. Amazon has some pretty good prices. Checked last night.
I gave my son an S-1 with black blade when he headed for Iraq. He thought it was too nice to risk there and bought a Ka-Bar USMC knife. Uses the S-1 now and an A-1 that he bought as hunting knives and for general utility on his land.
I once had lunch with Col. Rex Applegate and he told me that when he designed the Appegate-Fairbairn, he felt that a six -inch blade was enough. He based that on reports from the field when he was training OSS agents.
I never got used to the A-F sheath knife, but like all three sizes of the Gerber A-F folder. One is on my belt now, the big one. I think Boker makes the sheath knife.
Robert Terzuola used to make a very refined, improved version of the old M-3 Trench Knife, the M-30-A1 Gryphon. The Cutlery Shoppe arranged to have it made with an ATS-34 blade and a handle that Terzuola told me was very firmly attached. It handles very well and came with both bright and dull gray blades. I hope they still sell it. They also have a smaller Terzuola design, the M-10, I think. The blade resembles the Loveless semi-skinner. These were made to their specs in a very high quality shop in Seki City, Japan.
Many soldiers have relied on the Buck Model 119 or the longer Model 120. The latter is currently out of normal production, but some firms order them in special production runs. The Model 120 was the knife in the series of Scream movies. I've also seen it on some TV shows.
The biggest knife that I'd consider is the Fallkniven NL-2, the Odin model. Their fancier Northern Lights (NL) line have duralumin pommels, stainless guards and leather handles that much resemble Randall handles. All are named for Norse gods. The NL-2 has about an eight-inch blade. I use Kiwi brown or neutral shoe polish on the handle and did that on a Randall Model 3 that I once owned. It works well, but I'd sure get a Micarta, Zytel, or similar handle on a knife that might see service in a jungle or other humid area. Leather handles and holsters rot or mildew in such circumstances, although they hold up much better if dipped in a liquid designed to avoid mold and mildew. I think the USMC style Ka-Bars are so made.
Modern Ka-Bars don't show as good fitting as did the WWII originals, and the handles on the older ones are slimmer, and fit most hands better.
If I'd been in WWII, I'd have tried to get a Remington RH-36 hunting knife or the similar Marble's Ideal, with six-inch blade or buy an early Randall. The M-3 was also better than the Fairbairn-Sykes as both a combat knife and a utility model. The edge bevels are too thick on the Fairbairn, and a keen edge is hard to attain. The PAL RH-36 was made after Remington quit making knives in 1940, and was another good choice. The pommel is smaller than on the Remington version, but it's otherwise much the same knife.
The first Model Fairbairn (look it up) had a curved guard and solid ricasso on the blade to make it stronger near the guard. The Second and Third model don't have that, and some blades snapped. The tips will also break if the knife is thrown into a wooden wall. Don't ask how I learned this...
Click my link to the Fallkniven site. I think you'll be impressed. Work the buttons and see the various designs. Their F-1 is the issued knife to Swedish military pilots. Click on Exclusive Knives to see the NL line. PRICES ARE IN SWEDISH KRONER. They have US dealers who often sell for well under retail "list", but you won't get any really cheap. They are not inexpensive knives. But I think they're superb. Amazon.com has the A-1 for under $200 and the S-1 for about $168. They are excellent values, and
well under "list" price.
I haven't tried the Bark River knives, but they look good. The Smoke Jumper pictured in this thread looks to be a remarkable knife.
If you don't like the dip in the blade spine on the Randall Model 1, have them make the knife on a Model 5 blade blank. The blades are otherwise about the same, so a Model 5 blade sharpened on top is functionally a Model 1. It 's a superb all-round field knife, where legal. Be aware of laws about knives with a sharp top bevel.
Search on the Net for all the knives mentioned. You'll be impressed, I think.
And order a copy of Col. Applegate's, Kill or Get Killed. It'll do a basic job of teaching the use of the knife in battle. If you find Fairbairn's little book, it shows the critical arteries and tells how soon a stabbed or cut artery will cause death or incapacitation.
I've studied the use of the combat knife since I was 12, and feel pretty confident that any of the knives recommended here will serve you well.