What's the best choke for buckshot depends on:
The individual gun.
The individual choke.
The brand of shells.
The size of shot.
I have a friend who's "into" shotguns, and his Remington shoots #4 buckshot best from a Improved Cylinder and #00 buckshot from an Open Cylinder.
To complicate things, it shoots Winchester brand 00 from the Cylinder Bore tighter than Remington brand, but it's the other way around with #4 and the Improved.
Bottom line is this. Most manufactures have gone to the Improved Cylinder for buckshot guns.
To get the best patterns, you have to test a variety of brands of shells, to find the ones that work best in your individual gun.
If you already have a Modified gun, buy shells and test pattern until you find a combination that give good patterns from your gun.