On which hinge(s) should hinge pin doorstops be installed?
I do not know if there is a &quot;should&quot;.
I have always seen them on the top hinge, probably because it does not require bending over to install.
If you have a middle hinge that may be better as there will be less flexing of the door.
You have solid doors but I have seen them punch holes in hollow core doors.
I avoid them if i can, they can cause stress on  doors, trim and hinges- loosening/stripping hinge screws. I prefer baseboard or even floor mounted stops.