ORIGINAL: SwampCollie
That is just like what hair color on a lady produces the best wife (brunette) or the best girlfriend (blonde) or who makes the best truck (chevy) or even the best import truck (toyota).
But the answer is #6 shot. I've never used anything but #6s. Patterned plenty of everything, but #6s have always done me right. As long as you don't plan on shooting at extreme distances and shoot birds in the head/neck they will lay'em down.
Gotta shoot that gun and see what your gun likes, thats the only way to get an answer and not a bunch of opinions! Good Luck!
I will agree with SwampCollie on the #6 shot. Plus, when shooting #6 shot, you will get more pellets in the shell, resulting in more pellets flying at the bird. #6 shot won't always pattern the best...One thing about shotguns and patterning is there is never an always. If you and I have the same exact setup, my gun may shoot better than yours, or vice-versa. I have been talking with a former world record holder in still target shotgun shooting (Clark Bush), and he tests several shotshells with several different guns / chokes and he says that more often than not, a 3" #6 shot shotshell will produce the best evenly spread pattern at 40 yards. As far as brand of shell, most people have pretty good luck with the winchesters...particularly the Xtended Range. They are a High Density shotshell and cost around $30 to $35 a box.