I don't know if there is any optimal fighting knife. In most places, one cannot go down the street toting a big bowie, so in that case, it makes more sense to go with a folder.
I'll tell you what I think it would be, if you could afford one. The most lethal looking fixed blade I ever held was a genuine Bill Bagwell Helles Belle bowie. Ten inches of dangerous looking sharpness.
If I was going somewhere where I could carry a big blade, but not a handgun, and could afford it, I'd carry a bagwell.
In terms of folders, I like the Spyderco Military, the Endura, and the Emerson Commander. I also like the feel of the CRKT M-21, and the Cold Steel X2 Voyager, which I am meaning to buy one of in the near future.