There is no "best".
Every gun and every brand/type of shell produces different results.
Buckshot may produce VERY different results than slugs in the same gun.
The only way to find out, is to pattern a variety of ammo in your gun. If you have a gun with interchangeable chokes, you'll also have to try changing them to find what works.
You may well find a situation where your gun with one choke is good with buckshot, but another choke produces tighter groups with slugs.
To further complicate things, one brand of shot or slug may be much better than another brand in your gun.
IN GENERAL, most people have the best luck with the Improved Cylinder as a "all around" choke for both slugs and buckshot.