To be honest, Disney restaurants are now always booked and full because of the very popular dining plan and deluxe dining plan, not to mention everyone is starting to realize that you must make your reservations 6 months in advance and everything seems to be full all the time.....
but if you still want to try to go at a "off time", here it is:
Bfst is most busy from 8am to 9:30am
lunch is most busy from 11:30 to 1:30
dinner is most busy from 6:30 to 7:30
I would try to get a very early bfst time such as 6:30, lunch at 11am or later like 2:30pm, and dinner early or late such as 5pm or 9pm.
Of course, this won't always work especially if you have young children or if you have a set schedule.