If it's purely a buckshot gun, a bead is fine. A fiberoptic or tritium bead is even better.
If you have any possibility of slug use, ghost rings are fast and also accurate. A red dot is supposedly the tits according to everyone I've known who'se used one on a shotgun, but I have yet to (only rifles, SMGs and MGs).
I've used LPA (Benelli) MMC, XS and just acquired an 870P which has a winged ghost ring rear by Iron Sight Gunworks I think (never heard of them) and a winged bead front, which is a little odd.......going to see how it shoots.
The LPA is very fast and accurate (headshots from the prone at 100 were no big deal) and the XS is very fast. I like their white stripe if tritium is not available.
A good Surefire, either a fore-end of the handhelds attached, and a sling is about all you need. The SGA stock set, a Fury and a VTAC sling is all that's in future for this 870 of mine, unless I go with a folder (keeping it small is key in this particular gun's future).