To my mind, a defensive bear is 12g pump. Hands down, a good reliable, slicked up, short barreled 870.
Second choice, a short stocked, Marlin Guide gun in 45/70, 444 or 450 Marlin interchangeability.
If all I could choose was a H&R, ANY of your three choices. PROPERLY LOADED. Would be, IMHO, your best choices.
Like Cougar said, some slugs are better than others. Some years back there was a story of a brave gent who went buff hunting with a shotgun. He used Brenneke 3" Mag shells with special hardened projectiles. (Black Magic Magnum) They still make them today IIRC they are over 600g and produce near 1500 fps and over 4K FP of energy. That's a serious slug!!
In the 45/70 I would want 350RN Hornadys at 2250-2300 fps.
In the 444, probably a Barnes 300 SP at top vel (vs accuracy) Say 2100fps
In the 450, the same 350 RN Hornady bullet. 2100fps
The 500, 400g bullet at 2000 fps
In this "game" big bullets that disrupt the most tissue and drive deep to break down big bones will stop fights the quickest. YES accuracy matters too, but with practice comes consistancy. With consistancy accuracy and confidence. These are all things you must have to live thru such an encounter.
The single firearm, I rely on is my 870 with me running it.
CW