This is a good thread. It reminds me of something I read a little while back about some Russians. It seems they make a habit of hunting bears with knives. From what I recall, as a bear would stand up on its hind legs, they'll take a y-shaped branch/stick or something and prop it up by the neck just long enough to run in and gut it.

That takes some serious crazy.

As far as what knife is best? I have no idea and hope to never find out. The one I would use, however, would be the biggest one I had on me, but I would only use it after I ran out of ammunition.