I'd recommend around 10:30 for all breakfasts, especially if you schedule in the park that opens early that day.
You get the best of everything. You can go in early, ride the popular rides, get fast passes, then eat your breakfast with the restaurant starting to empty.
For instance, last July we had a 10:30 at play and dine. My bro and I went to the park early, rode TOT and Rock-N-Roller, then Star Tours. We could have rode those again if we felt like it. We then hung out on a bench until my wife, daughter and the rest of our group came to the park.
The restaurant was practically empty and the characters were constantly at our table. The kid's loved it! To boot, the food was still being brought out fresh and was very good.
From now on, all our character breakfasts will be at that time because you have so much freedom.