I read an article once written by a guy who hunted really large grizzlies with a .500 S&W. He stalked the bear to get (too) close, and then had to empty the revolver into the bear before it fell. He also had a guy with a big-bore rifle for backup.
That said, you can't kill something if you can't hit it.
So, the correct answer is the biggest, most powerful cartridge you can shoot well enough to hit a bear with. Anything less will leave you wanting more, and anything more won't serve you well if you can't hit the bear with it.
Daryl