Shotguns
Most painful to shoot were the single shot Stevens and H&Rs. Both kicked like mules and made my teeth rattle. The most painful round out of my Mossberg 500 was a 3in. turkey load that a so called "friend" said go ahead and shoot it. I did not thinking anything about it. It hurt so bad that I had to set the gun down while my friends were rolling on the ground laughing. I had a purple stain that ran from my shoulder, down 4 inches along the rib cage from the armpit, and on the inside of my right bicep to the elbow. My wife saw it when I got home and wondered what did I do to myself.
Rifle
The most recoil from any rifle that I've shot was a Number Three Ruger in 45/70. It was a friend's gun with two loads. The 300 grain bullet loads weren't horrible, but I really had to brace myself for the 500 grain loads that he had. Felt like the push of a 12 gauge mag combined with the snap of a 30.06 bolt rifle.
Handgun
The most painful was shooting some hot 300 gr., 44 mag. handloads some friends were going to use boar hunting. Shot them out of the one friend's Colt Anaconda, and they were bearable. Shot them out of the other friend's 5 in. Ruger Redhawk, and the rubber grips tore the skin off the webbing of my right hand from the recoil. These were the same guys by the way that had the 3 in. turkey load.
Needless to say, I don't own a rifle chambered for anything more than a 30.06; I won't shoot anything bigger than 2 3/4 in. 00 buck or slugs out of a 12 ga. pump; I'm not a 44 mag recoil fan, so I'll stick with my .357 magnum whimp revolvers. Call me a chicken, but I think that I'm older and wiser.