WOW! Lively board and I feel I must put in my 2 cents.
1st let me say I do not profess to be an expert just somebody that reads a lot.
To anwser the main question at hand, what knife, I always carry a large 9-10" fixed blade with me and that would be my vote of what to use.
I will say that I would never consider a krambit for this task nor would a kukri be my first choice either. Reason being these are slicing and choping knives and greatly limit your type of attack.
The krambit you have to wait until its on top of you (not a good place to be) and the kukhri is really only good for choping (kuk fans please don't get on my case for saying this. Yes, I know you could stab something with them but its not what they are desinged for and to really sink the tip in you still need to move the balde in an arcing fashion) and while I know that a kuk could lop a limb off think off trying to chop something (& make effective contact) moving at 35mph and thats if you see it coming.
As far as how to defend against a big cat attack lets face it. If its a lion or tiger you pretty much have no chance these are animals that crush you with a bat of their paw just from the impact force and can easily fit your head in their mouth and crush your skull just by bitting down (yes the do have the bit pressure capable of this).
With any of the big cats in the americas you might have a chance. Also it is important to know how a cat attacks. It is a stealth hunter. It stalks, so if it can its going to attack from behind, or more preferably to mt lions above.
It is important to know that big cats don't really attack or try to kill with their claws. They do this with their teeth and very powerful jaws, preferably breaking the neck. The claws are so they can hold on to you while the do this. Any collateral damage done by claws is just a bonus.
For those that comment on killing the cat after they get it off of their backpack with lines like "it won't be on my back the whole time and then when i get it off me i kill it", no, keep it on your back. That backpack is the only thing keeping you alive. At that point tuck your head and arms in as much as possible and wait for the people you're with to try and help you...because lets face it... your smarts are your best suvival tool and going anywhere alone is always stupid.
If I am lucky enough to see the cat coming, what I would do is be big, loud and posture myself and if it goes I'd keep my weapon, what ever it may be, out and ready until i'm somewhere there are more people than wilderness.
Trust me, its not gone. Cats are tenacious hunters. If my postureing doesn't work I'm going to hold out my hiking pole like a pike or extend my arm with blade in hand and get a strong foot stance and just wait for the leap (cause it will leap for the neck that what big cats do that's how they kill.) and hope that I stab it somewhere that does some damage. Now some may say that this wouldn't work because a cat wouldn't just run into yout knife. Well a cat doesn't know what a knife is and cats actually have bad depth perception. If you keep your arm pointed directly at them they shouldn't be able to see how far it is extending.
Now all that said I never would want to face big cat with just a knife or a gun for that matter. As others have said you need to make that first shot or two count cause if not you're screwed.
As far as what I would want with me, I would have to say a very large can of pepper spray the kind that they sell for bear attacks. Why? a couple of reasons: 1st you don't have to be as accurate as you do with a gun and it still gives you some good range.
2nd, if its on your back you don't have to worry about blade length and tyring to stab/hack at it and yes in that proximity you will get yourself too but I think it'll be worth it.
3rd, you can carry pepper spray where you can't carry a gun like in many state parks.
4th, if this stuff hurts you think how much more sensitive an animals senses are and how much it will hurt them.
5th & last reason for pepperspray, I hate people that kill animals for being animals. Yes, I know if its life or death I'd prefer the animals death to mine, but if I don't have to kill it, why should I.
Now, I'm sure this long ranting and raving post will get some hornets buzzing, but I'm hoping that it will also make alot of sense to most everyone.
Cheers
-Ronan