I've never been quite sure what a "fighting" knife is, other than any knife you use in a fight. For instance, the Gerber Mark II looks badass, but I suspect that it's a lot less useful than, say, the KBAR that's been mentioned here for most uses a knife gets put through in any sort of combat zone.
My thought is that, before picking the knife, decide what style or method of knife fighting you will use and/or train in. That will often dictate the general requirments of the knife.
The KBAR is a good choice, but not because of how "deadly" it may be, but because it is also well suited to function as a general purpose utility knife.
Is this something you plan on carrying? If so, you should buy something that is legal to carry in you area -- (1) why buy a self defense knife that won't be with you all the time, and (2) if you ever have to use it, you will come out a lot better under the law if it's legal.
My two main fixed blades I carry for self defense purposes are a benchmade outbounder and a benchmade nimvarus cub (the nimvarus might be a good choice). I believe that a 4" blade will be sufficient for any sort of self defense purpose I might have.
Now, if you want something to repel the hordes of zombies breaking through the wire, get something big, badass, and swingable.