Take your pick
You would get a lot of different answers on this one; I'm not the "Duck Commander" so here is my style and preferences:
I only shoot over decoys (20-40 yards), I never pass-shoot until the last day of the season. I river hunt out of a jon-boat. Clean-kills only as I have no dog yet. I usually shoot them as they stall or flair. Works good on Mallards, Spoonies and Teal. Not so good on Pintails as the drakes usually hide back or tight in the flocks behind the hens so its hard for me to pick them out.
With that said..
I shoot 3" # 2, usually Fast-Steel and 2 3/4 #2 Remington. I like to get more than 1,400 fps but OK with more than 1,200. I use 3" BB for any cripples in the water. I don't shoot geese any more. I load the 2 3/4 in the barrel and next in the mag, then the 3" is my 3rd shot.
I shoot a variety of shotguns, my everyday gun is a Hatsan Escort; I use the modified choke. What I get are ducks that fall straight down and die. I shoot an O/U as a back up and I only shoot 2 3/4 with it, Mod for 1st shot and Full for 2nd shot.
****Word of caution****
I used to shoot my 870 (made in 1985, a Wingmaster), 26" barrel with full choke using shells like "Heavy shot", "matrix", "tungsten" etc. I peeled back the muzzle like a banana one day. No lie, just like the cartoons, 6 to 8" back.
It was not mud in the muzzle either..
I sent some pieces of the choke to Remington; they told me that my choke tube did not have a rating for steel and to never use full chokes with that stuff that won't roll in the wad,
Even though it had "steel" stamped on it.
There will be some that would differ or question my claim (that's OK) but I promise you, once it happens to you...you will see.
So, I leave my full chokes at home.
Hope this helps.