Ok. In a hand to hand fight, something like the Ontario Bagwell Bowie would be ideal.
Otherwise, these would be my choices:
Large Knives - SOG Bowie, Ka-Bar USMC, or SOG Daggert
Medium Knives - SOG Seal Pup, Cold Steel Desperado, Boker Applegate Fairbairn.
Small Knives - Cold Steel Safe Keeper, Benchmade Outbounder, Emerson Knives fixed Karambit.
The type of knife you fight with is not really important. You can fight with any knife. Choose several knives and know how to use them properly. It is better to have more than one regular knife than a single "perfect" fighting blade.
No matter what you choose, remember this:
What is important is experience, skill, speed, reflexes, and judgement. Using a knife against another person is brutal, bloody, sickening business; be prepared for the consequences. If you can't use a knife properly, it does not matter what kind of "fighting knife" you have. Above all, knowing when and how to avoid a fight is the best defense.
Sorry for rambling,
TheSurvivalist