I have used them all #71/2 to #4 using the same 12 ga. gun with full choke, and I settled on #5. I find that most of the time, there's hardly any pellets spoiling the meat.
Having a good retriever helps also.
Lately I've been using a 20 ga. single modifed choke and I find #4 works for that better. I even have to wait and shoot later as the gun points really fast being very light.