375 H&H muzzle energy ~4500 ft/lbs. 12 gauge slugs up to 3000, somewhat lower but probably enough to knock a bear on his ass. If you hit him.
When my brother and his family went rafting in AK, they took a 10 gauge with magnum slugs. He figured that was about as powerful as most rifles at close range, and he wasn’t going to be shooting at any bears across the valley.
10 gauge is the largest smooth bore allowed these days I think. I hear that fishing guides carry 12 gauges with slugs and stand behind their fishing party guarding against a bear ambush. Sounds like a good choice too. I am reasonably good with the big bore rifles so I’d opt for the 375 but even something as lower powered like a 30-06 with a 220 gr load should be effective. You tangentially mention the best round is the one you can hit the target with at close range. I agree, while a 460 Weatherby would take him out, few can fire that gun proficiently enough to be comfortable in the knowledge they could hit the bear at close range!