Broadside exposure, I feel the lungs provide the largest lethal target.
Seems a lot of bear guides prefer their clients aim for shoulders, to break/bring the bear down where it's hit. Allowing for additional follow-up shots to the finish the bear off while it's out in open terrain. Which are often necessary, because shoulder shots don't always damage vital organs. Of course the guides are also blazing away after your first shot, so they don't have to follow the bear into the brush to retrieve it. Can't blame them... I'd probably have the same preference if I was a guide, having to clean up after my clientele. Some of which can shoot... some which can't. I think the guides prefer the shoulders because if their client's shot is bad, they get instant notification. If the bear doesn't struggle to stay afoot after the client's first shot, they know it's time to pour in more lead.
When I hunt by myself or with a buddy, I aim just behind their shoulders, waiting for broadside shot profiles. Same as I would if archery hunting. Last Kodiak bear I shot trotted less than 40yds after shot behind his shoulders. Some lung-shot bears have dropped dead in their tracks. None traveled far, if the bullet expanded properly while traveling thru their lungs.
My experiences are with 338 caliber bullets traveling at healthy speeds. From 338 Edge and Lapua - Improved class cartridges.