I know I'm probably in the minority for suggesting it, but the .357 has given the best chance of walking away from a bear attack historically. One of the universities up here (I forget which) looked at the data involving firearm use against bear attacks. They found that 1.) Bear spray is 90% effective at stopping a bear attack, beating every firearm category, and 2.) The weapon with the best percentage of stopping an attack before an injury (to the human
) was the .357. I believe the 30-06 was next. The .44 and bigger categories didn't fare so well.
It's been said previously - the best is whatever you can shoot effectively. The study inferred the folks running around with the bigger guns couldn't shoot them effectively, while someone with a .357 was able to shoot a quality round and PLACE IT EFFECTIVELY.
It goes back to the Inuit woman who took down the polar bear with a .22lr - she put the round through the eye and into the brain cavity. Not that I'd recommend recreating the feat, but hitting your target with an adequately-built round seems a lot more important than shooting a grizzly bear in the tail with a .454!