When I think of fighting knives, I think of bowies, daggers (like Fairbairn-Sykes, Gerber Mark II), Rambo/Special forces style knives, bayonets, and daggers (Green Beret knives, old SOG offerings, military bayonets, trench knives)
For fighters, there's basically the need for length. (To reach vital organs, and for general ability to reach further) Good balance for fighting and quick transitions is also important. (Some fighting knives just feel electric in the hand... Really fast. So cool to feel) I guess for military use though, some more tougher and utilitarian blade shapes can be useful.
Some knives will be more tool use oriented, some will be more killing humans oriented.
I like stuff like:
Old SOG knives from the nineties, seki made
Old Al Mar knives
All sorts of custom bowies
Fairbairn-Sykes daggers
Applegate-Fairbairn knives
Gerber Mark II,
Chris Reeves/Harsey Green Beret knife
All of the above can have some degree of collectibility and make good additions and representations of the fighting knife category. Collectibility is almost all about rarity vs desireability/popularity vs personal/historical value of the knives. My own desires and interests lead me to value the knives above. Yours may lead you to different areas.