Answer:
The most powerful in terms of muzzle energy is probably the .950 JDJ. The rifle weighs 80-120 lbs and even with ports and an 18 lb muzzle break with 4 mercury recoil reducers the gun still has 200+ ft lbs of recoil.
.950 JDJ 3,600 grain 2,200 fps 38,000 ft lbs of energy.
The most powerful shoulder fired cartridge is the 700 Nitro Express with a 20 lb rifle and still 160-200+ ft lbs of recoil.
700 Nitro Express 1,000 grain 2,000-2,600 fps 8,900-15,500 ft lbs of muzzle energy.
The most powerful sporting rifle/cartridge also goes to the 700 Nitro Express.
If you count black powder elephant rifles as sporting cartridges then its the 2 bore with its 1.32 in. muzzle diameter and its low muzzle velocity keeps it virtually painless and yet kicks slightly more than the 700 Nitro express.
S&H 2 bore 3,600 grain 1,500 fps 17,000 ft lbs