There is no right answer to your question, because every gun is different, and what proves to be a very accurate slug load in one can be very poor in the next, and vice-versa.

In order to determine what shoots best in your slug barrel, I'm sorry to have to inform you that you will have to ACTUALLY SHOOT IT at a range where you can put targets at known distances and shoot from some kind of a rest in order to eliminate human error as much as possible. This is a painful exercise with slugs, as they kick like an African dangerous game rifle!

However, over the years I have noted that Brenneke slugs and the similar Lightfield type are often quite accurate in the 870's. Try several styles/brands, and I'm sure you will find one load that is notably more accurate than other types. I have found that the Brenneke's and Lightfields also evidence considerably better killing power than the lightweight, hollow-base Foster type slugs, which are pretty poor killers despite their impressive diameters.

(IF you're still giggling after shooting severalboxes of slugs from thebench, you're a better man than I am, Gunga Dhin!)