The best days depend on time of year you are going and whether you are staying on site to qualify for the Early Magic Hour.
Either way, one plan of attack is to follow the day after theory. Go to the parks the day after they have had EMH. So many resort guests will have just attended that park the day before that it won't be as crowded. The exception to this would be Sunday at MK. Sunday is MK's EMH. That would probably be worth going to, and Monday is not a good day to go to MK because so many tourists are starting their vacation at that park.
If you are going during a busy time of year, I would definitely go first thing in the morning.