Some scopes can cost more than the Air Rifle. Leapers makes some excellent scopes for around $250. Whatever scope you purchase make sure it is designed for air rifles. Air rifles, because of their forward / reverse kick, can break a regular hunting scope not designed for this type of rifle.
TASCO
The best you can afford.
Best left to a good gunsmith.
Impossible to answer without knowing what your end state is. Even then, "best" depends on your pocket book, skills, rifle, etc..
Best left to a gunsmith
Either the barrel or receiver would have to be drilled and tapped or some form of barrel clamp would be required to hold the scope. At this point, given that the rifle is over 75 years old and no longer manufactured (read: parts are not available off the shelf and manufacture of same is expensive and difficult), your best choice would be to not do it.
The m1 garand for its power and it can have a sniper scope
Best left to a gunsmith
Assuming you mean the Remington 870 shotgun, with a scope- is your barrel rifled? If so, sabot slugs are generally the most accurate. If smoothbore, then rifled slugs will be most accurate.
One that was designed for air rifles. Best will be determined by your intended use and how much you spend.
"Best" is a subjective term that will be based on your pocketbook and what you are using the rifle for.
Nikon Prostaff is an excellent variable scope for under $250. Excellent optics, can't beat it for the price.