I would classify a fighting knife as something that was issued or private purchased in sufficient numbers to be recognized. The one on the left appears to be a survival machete, no idea what the other four even are.
As for classification, blade length a consideration. I'm thinking 6 inch is the smallest for a traditional fighting knife but that's not set in stone.
The center knives with great notches wouldn't be well accepted as they'd get stuck in everything. That has been passed on from guys that used knives for their purpose at war. The two others would probably be fine but all of them are Gerber knives that were never tested to any extent. All long since discontinued, they're collector pieces that fetch far more than they ever cost. Gerber made ONE as far as I know, the Mk2 that was an accepted fighting knife...
Thought I'd throw some more pics here. I grabbed one each of the newer Gerber knives mainly because I wished they'd been available 50 years ago...I think they're about perfect for issue. Not too big but certainly serviceable. The US has issued some, depends on who and where would dictate which you got I guess. I've seen troops overseas doing vids on different subjects while in possession of them. I don't agree that a knife has to be huge for combat, matter of fact a reasonable size is better for general use. Look back to WW2 and all the variants. From left here...Gerber strongarm, prodigy, LMF 2 Infantry.
IMHO true "fighting" knives fall into a few narrow categories such as the Fairburn Sykes Fighting Knife issued to British Commonwealth Airborne troops and Commandos or the Knife, Fighting, Commando Type, V-42 issued to members of the FSSF. Both of those weapons are daggers. The Ka-Bar Knife issued to the USMC is one, and of course few would argue that the Kukri in the hands of a Ghurka wasn't a formidable fighting knife but most of the rest are utility knives or machetes not fighting knives. There have been many fanciful knives offered as fighting knives from various sources such as the over the top Buck No. 184 "Buckmaster" but everyone knows it isn't a true fighting knife.
I second that most are utility knives. Soldiers need knives but not really for fighting. With the exception of elite forces who needed stealth, it's probable very few were used in combat.
But there's something about a good knife, just feels good having one. My personal favorite is the Ontario pilots survival knife. Been carrying one deer hunting for over 30 years. It's not huge but fits my hand perfectly.