ORIGINAL: sjsfire
You'll ahve to buy a couple of different brands and try for yourself. No two guns shoot the same. I will say in my 870 smoothbore the Winchester XX slugs shoot the best.
+1
I have an 870
(doesn't everyone? 
) and have tried a bunch of different ammo. The best groups I got were from Federal Tru-ball rifled slugs. But then like sjsfire was getting at, this was from my 870. Your 870 might not like Federal Tru-ball.