I've run all over the place with barrel length over the past few years. Regret selling some of them, ended up with doubles of others. The 870 platform, like most popular remington platforms, offer a ton of choices.
The 8.5 (grizzly barrel) was too short imo. Felt like I was going to lose fingers every time I had it out.
The 12.5 was my go to for a bush defence length. Wish I had of kept it.
The 14 is great. I currently have a 14" on my backpacking 870. I think the 18" is the best all around. I have one set up in a sling that hangs a bit low but very useful all around.
To answer your original post OP, I have a base 870 with a 14, 21 w turkey chokes and a 28 with chokes. Those 3 seem to cover everything I need out of a pump action.
Everything shorter than a 14 seems to be very loud and doesn't group well. Also I have never appreciated rifle sites on a shot gun. To each their own I guess. Some guys love em. Also the rifled barrels aren't my bag either.