"So what kind of knife would you use against a bear."
If I had to try to fend off a bear with a knife, I guess I'd pick a Busse 8" Pure Bred Fighter: fast, accurate, good penetration, strong enough not to snap when trying to penetrate. Of course, this is a situation I'd rather avoid. I'm under no illusion that it would be easy to stop a bear with a knife.
By the way, if you read Bear Attacks: Their Causes and Avoidance (by Stephen Herrero), Alaska Bear Tales (by Larry Kaniut), and Shooting Bears (by John Pazzenti), you will see that most firearms are also quite unreliable in stopping a large, determined and/or enraged bear. A large bear will have a thick enough skull that bullets will often bounce off, or lodge in the skull, stopping short of the brain. A really big bear will also have a tough enough hide over a thick enough fat layer that bullets will often stop short of penetrating vital organs. Even a successful, mortal shot is unlikely to stop a bear's attack until it bleeds out (which is likely to give the bear plenty of time to finish mauling you).
On the other hand, there have been some cases of (doubtless, tougher than I) people killing bears with knives, or even sticks. And let's not forget that our prehistoric forebears successfully fended off bears with stone and wood weapons, many, many times.
--Mike