Most punishing was a friends single shot (daayum that thing's light) 10ga turkey gun with 3 1/2 inch magnum turkey load! First round produced immediate headache, but with him cackling "wanna shoot it again?? want ta shoot it again?" and nearly dancing around me,,, the only answer was h&!! yes! NEVER do that again!
Most unpleasant rifle was an older Ruger .44 auto. Not at all powerful, but a really unpleasant "jab", don't know quite how to explain it.
44mag pistols aren't a big deal, most uncomfortabe had to have been a Colt .38 detective special 2" with hot handloads.
Bob