I shoot both 12 and 20 gauges, I recently started shooting a 16 gauge, and I prefer 7.5 shot in all three gauges. It's been extremely reliable over the years. The best advice I can give, is to try 7.5's and 8's or 9's, on a patterning board, the skeet range or whatever shotgun game you like to shoot and see what works best for you.

Good Luck