Toss-up between a Browning B78 .45/70 with the tomahawk buttplate and an Ithaca Super Single slug gun.
Honorable mention goes to my uncle's 20" Deerslayer. Recoil was manageable, but the K4 in pivot mounts scarred many unwary folks who just had to try out that newfangled scoped shotgun.
Have a light 10 ga single shot, lighter than NEF but not sure of the maker anymore... with heavy turkey loads it still wasn't bad at all.
Wife and I have shot all the way up to 460 wtby.
To this day the horror remains a 378 wtby a friend had.
It doesn't fit the OP question really, as the recoil was manageable totally BUT it hit you so quickly it was brutal there... slow those foot pounds down just a bit and it might be ok.
Broke my buddies shooting glasses in half on the first shot when it shoved something into them, likely his thumb...
I"ve had others that kicked like a mule, a buddies Rem 78 in 06. I"d rather shoot my MK5 300 wtby, both without brakes..
We can keep Larry Root and all his idiotic blabber and user names on here, but we can't get Ralph back..... Whiskey Tango Foxtrot, over....
For sure a single shot H&R, shooting a 3 1/2" turkey load. Rough but doable.
Another vote from here!
Founder Ancient Order of the 1895 Winchester
"Come, shall we go and kill us venison? And yet it irks me the poor dappled fools, Being native burghers of this desert city, Should in their own confines with forked heads Have their round haunches gored."
Not the hardest kicking gun I ever shot but one that kicked all out of proportion to what it should have was a Remington 700 ADL wood stock in .270. It just hurt. It didn't wallop you, it just was painful to shoot.
Maybe not the worst but I sure remember it. The first centerfire rifle I ever shot; an 03-A3, 30-06 with 220 gr bullets from prone position when I was 12 years old.