I use 7 1/2 for everything. I've found it to have the optimal mix of dense pattern, reliable quick kills, bone breaking power at all practical upland shotgunning distances, and without an excessive number of pellets to pick out. In 20 gauge, use the cheap game loads in the early season with an IC or Mod choke. Later in the season switch to Federal Premium for an extra five yards of effective distance.
If it were just about killing the birds quickly, I'd use #9. It's deadly because the probability of multiple hits in vital areas is high -- almost 600 pellets per ounce versus 350 for 7 1/2.