I'm thinking .45-70 just because the sectional density (and therefore penetration ability) is better than MOST slugs. But then you've got DixieSlugs & the like, which are known hammers. But it'd be tough to top a 500-gr Buffalo Bore .45-70 hardcast load, in terms of penetration ability, and that's what you need for big bears, in case you hit skull or other big bone on the way in. .50 Alaskan or .500 Phantom might be better than either, however. Seven rounds semi-auto, of a 570 grain, .510 cal load from a .500 Phantom AR10, at around 1688 fps, or a 750 grainer at about 1,120 fps, would make for a serious bear-stopper.