.357 minimum. 44 magnum really.
Lots of different calibers have worked. The most popular is the .44 magnum.
Guides in bear country carry 12 gauge with slugs.
.357 Buffalo Bore is reported to drill a hole 4.5 feet through Mr. Bear.
That seems sufficient.
50 or 60 years ago that was the conventional wisdom of the day. That's no longer the case. Today the 10mm Glock 20 loaded with Buffalo Bore 220gr is the go-to bear defense pistol. If you carry a revolver for bear defense in Alaska you're probably at least 80 years old.