Hope for the best...PLAN for the worst....
Like others say it really depends on the style of hunt you are going to be involved in.
If you have fairly open terrain with moderate yardages and are going to shoot the bear, when it is relatively unaware of your existence, then most any suitable big game caliber with a good bullet will do the job.
If you are hunting in close terrain, where up close and personal bear encounters may be the name of the game, then the medium calibers (.338 - .375) and heavies (.416 - .458) with tough bullets designed for deep penetration, capable of reaching where they live when shot from almost any angle and breaking major bones while doing it are going to be you best chance of coming home to tell the tale of your latest hunt.
The guides may be carrying a .375 H&H, .338 Mag, .458 Mag or even a .416 Rigby or a heavy loaded .45/70 in a Marlin guide gun, but in a charge situation the more firepower that can be brought to bear
the more likely the hunt will end successfully.